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The top list of academic search engines

1. Google Scholar

4. science.gov, 5. semantic scholar, 6. baidu scholar, get the most out of academic search engines, frequently asked questions about academic search engines, related articles.

Academic search engines have become the number one resource to turn to in order to find research papers and other scholarly sources. While classic academic databases like Web of Science and Scopus are locked behind paywalls, Google Scholar and others can be accessed free of charge. In order to help you get your research done fast, we have compiled the top list of free academic search engines.

Google Scholar is the clear number one when it comes to academic search engines. It's the power of Google searches applied to research papers and patents. It not only lets you find research papers for all academic disciplines for free but also often provides links to full-text PDF files.

• Coverage: approx. 200 million articles
• Abstracts: only a snippet of the abstract is available
• Related articles: ✔
• References: ✔
• Cited by: ✔
• Links to full text: ✔
• Export formats: APA, MLA, Chicago, Harvard, Vancouver, RIS, BibTeX

BASE is hosted at Bielefeld University in Germany. That is also where its name stems from (Bielefeld Academic Search Engine).

• Coverage: approx. 136 million articles (contains duplicates)
• Abstracts: ✔
• Related articles: ✘
• References: ✘
• Cited by: ✘
• Export formats: RIS, BibTeX

CORE is an academic search engine dedicated to open-access research papers. For each search result, a link to the full-text PDF or full-text web page is provided.

• Coverage: approx. 136 million articles
• Links to full text: ✔ (all articles in CORE are open access)
• Export formats: BibTeX

Science.gov is a fantastic resource as it bundles and offers free access to search results from more than 15 U.S. federal agencies. There is no need anymore to query all those resources separately!

• Coverage: approx. 200 million articles and reports
• Links to full text: ✔ (available for some databases)
• Export formats: APA, MLA, RIS, BibTeX (available for some databases)

Semantic Scholar is the new kid on the block. Its mission is to provide more relevant and impactful search results using AI-powered algorithms that find hidden connections and links between research topics.

• Coverage: approx. 40 million articles
• Export formats: APA, MLA, Chicago, BibTeX

Although Baidu Scholar's interface is in Chinese, its index contains research papers in English as well as Chinese.

• Coverage: no detailed statistics available, approx. 100 million articles
• Abstracts: only snippets of the abstract are available
• Export formats: APA, MLA, RIS, BibTeX

RefSeek searches more than one billion documents from academic and organizational websites. Its clean interface makes it especially easy to use for students and new researchers.

• Coverage: no detailed statistics available, approx. 1 billion documents
• Abstracts: only snippets of the article are available
• Export formats: not available

Consider using a reference manager like Paperpile to save, organize, and cite your references. Paperpile integrates with Google Scholar and many popular databases, so you can save references and PDFs directly to your library using the Paperpile buttons:

Google Scholar is an academic search engine, and it is the clear number one when it comes to academic search engines. It's the power of Google searches applied to research papers and patents. It not only let's you find research papers for all academic disciplines for free, but also often provides links to full text PDF file.

Semantic Scholar is a free, AI-powered research tool for scientific literature developed at the Allen Institute for AI. Sematic Scholar was publicly released in 2015 and uses advances in natural language processing to provide summaries for scholarly papers.

BASE , as its name suggest is an academic search engine. It is hosted at Bielefeld University in Germany and that's where it name stems from (Bielefeld Academic Search Engine).

CORE is an academic search engine dedicated to open access research papers. For each search result a link to the full text PDF or full text web page is provided.

Science.gov is a fantastic resource as it bundles and offers free access to search results from more than 15 U.S. federal agencies. There is no need any more to query all those resources separately!

Where to find peer reviewed articles for research

This is our ultimate guide to helping you get familiar with your research field and find peer reviewed articles in the Web of Science™. It forms part of our Research Smarter series. 

Finding relevant research and journal articles in your field is critical to a successful research project. Unfortunately, it can be one of the hardest, most time-consuming challenges for academics.

This blog outlines how you can leverage the Web of Science citation network to complete an in-depth, comprehensive search for literature. We share insights about how you can find a research paper and quickly assess its impact. We also explain how to create alerts to keep track of new papers in your field – whether you’re new to the topic or about to embark on a literature review.

• Choosing research databases for your search
• Where to find peer reviewed articles? Master the keyword search
• Filter your results and analyze for trends
• Explore the citation network
• Save your searches and set up alerts for new journal articles

1. Choosing research databases for your search

The myriad search engines, research databases and data repositories all differ in reliability, relevancy and organization of data. This can make it tricky to navigate and assess what’s best for your research at hand.

The Web of Science stands out the most powerful and trusted citation database. It helps you connect ideas and advance scientific research across all fields and disciplines. This is made possible with best-in-class publication and citation data for confident discovery and assessment of journal articles. The Web of Science is also publisher-neutral, carefully-curated by a team of expert editors and consists of 19 different research databases.

The Web of Science Core Collection™ is the single most authoritative source for how to find research articles, discover top authors , and relevant journals . It only includes journals that have met rigorous quality and impact criteria, and it captures billions of cited references from globally significant journals, books and proceedings ( check out its coverage ). Researchers and organizations use this research database regularly to track ideas across disciplines and time.

Explore the Web of Science Core Collection

We recommend spending time exploring the Core Collection specifically because its advanced citation network features are unparalleled. If you are looking to do an exhaustive search of a specific field, you might want to switch to one of the field-specific databases like MEDLINE and INSPEC. You can also select “All databases” from the drop-down box on the main search page. This will cover all research databases your institution subscribes to. IF you are still unsure about where to find scholarly journal articles, you can learn more in our Quick Reference Guide, here, or try it out today.

“We recommend spending time exploring the Core Collection specifically because its advanced citation network features are unparalleled.”

2. Where to find peer reviewed articles? Master the keyword search

A great deal of care and consideration is needed to find peer review articles for research. It starts with your keyword search.

Your chosen keywords or search phrases cannot be too inclusive or limiting. They also require constant iteration as you become more familiar with your research field. Watch this video on search tips to learn more:

It’s worth noting that a repeated keyword search in the same Web of Science database will retrieve almost identical results every time, save for newly-indexed research. Not all research databases do this. If you are conducting a literature review and require a reproducible keyword search, it is best to steer clear of certain databases. For example, a research database that lacks overall transparency or frequently changes its search algorithm may be detrimental to your research.

3. Filter your search results and analyze trends

Group, rank and analyze the research articles in your search results to optimize the relevancy and efficiency of your efforts. In the Web of Science, researchers can cut through the data in a number of creative ways. This will help you when you’re stuck wondering where to find peer reviewed articles, journals and authors. The filter and refine tools , as well as the Analyze Results feature, are all at your disposal for this.

“Group, rank and analyze the research papers in your search results to optimize the relevancy and efficiency of your efforts.”

Filter and Refine tools in the Web of Science

You can opt for basic filter and refine tools in the Web of Science. These include subject category, publication date and open access within your search results. You can also filter by highly-cited research and hot research papers. A hot paper is a journal article that has accumulated rapid and significant numbers of citations over a short period of time.

The Analyze Results tool does much of this and more. It provides an interactive visualization of your results by the most prolific author, institution and funding agency, for example. This, combined, will help you understand trends across your field.

4. Explore the citation network

Keyword searches are essentially an a priori view of the literature. Citation-based searching, on the other hand, leads to “systematic serendipity”. This term was used by Eugene Garfield, the founder of Web of Science. New scientific developments are linked to the global sphere of human knowledge through the citation network. The constantly evolving connections link ideas and lead to systematic serendipity, allowing for all sorts of surprising discoveries.

Exploring the citation network helps you to:

• Identify a seminal research paper in any field. Pay attention to the number of times a journal article is cited to achieve this.
• Track the advancement of research as it progresses over time by analyzing the research papers that cite the original source. This will also help you catch retractions and corrections to research.
• Track the evolution of a research paper backward in time by tracking the work that a particular journal article cites.
• View related references. A research paper may share citations with another piece of work (calculated from bibliographic coupling). That means it’s likely discussing a similar topic.

Visualizing the history discoveries in the citation network

The Web of Science Core Collection indexes every piece of content cover-to-cover. This creates a complete and certain view of more than 115 years of the highest-quality journal articles. The depth of coverage enables you to uncover the historical trail of a research paper in your field. By doing so, it helps you visualize how discoveries unfold through time. You can also learn where they might branch off into new areas of research.  Achieve this in your search by ordering your result set by date of publication.

As PhD student Rachel Ragnhild Carlson (Stanford University) recently wrote in a column for Nature: [1]

”As a PhD student, I’ve learnt to rely not just on my Web of Science research but on numerous conversations with seasoned experts. And I make sure that my reading includes literature from previous decades, which often doesn’t rise to the top of a web search. This practice is reinforced by mentors in my lab, who often find research gems by filtering explicitly for studies greater than ten years old.”

5. Save your search and set up alerts for new journal articles

Save time and keep abreast of new journal articles in your field by saving your searches and setting up email alerts . This means you can return to your search at any time. You can also stay up-to-date about a new research paper included in your search result. This will help you find an article more easily in the future. Head over to Web of Science to try it out today.

“Everyone should set up email alerts with keywords for PubMed, Web of Science, etc. Those keyword lists will evolve and be fine-tuned over time. However, it really helps to get an idea of recent publications.” Thorbjörn Sievert , PhD student, University of Jyväskylä

[1] Ragnhild Carlson, R. 2020 ‘How Trump’s embattled environment agency prepared me for a PhD’, Nature 579, 458

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A search engine is a device that sends out inquiries to sites on the Web and catalogs any Web site it encounters, without evaluating it. Methods of inquiry differ from search engine to search engine, so the results reported by each one will also differ. Search engines maintain an incredibly large number of sites in their archives, so you must limit your search terms in order to avoid becoming overwhelmed by an unmanageable number of responses.

Search engines are good for finding sources for well-defined topics. Typing in a general term such as "education" or "Shakespeare" will bring back far too many results, but by narrowing your topic, you can get the kind (and amount) of information that you need.

• Go to Google (a search engine)
• Type in a general term ("education")
• Add modifiers to further define and narrow your topic ("rural education Indiana")
• Be as specific as you can ("rural education Indiana elementary school")
• Submit your search.

Adjust your search based upon the number of responses you receive (if you get too few responses, submit a more general search; if you get too many, add more modifiers).

Learn how the search engine works

Read the instructions and FAQs located on the search engine to learn how that particular site works. Each search engine is slightly different, and a few minutes learning how to use the site properly will save you large amounts of time and prevent useless searching.

Each search engine has different advantages. Google is one of the largest search engines, followed closely by MSN and Yahoo . This means that these three search engines will search a larger portion of the Internet than other search engines. Lycos allows you to search by region, language, and date. Ask allows you to phrase your search terms in the form of a question. It is wise to search through multiple search engines to find the most available information.

Select your terms carefully

Using inexact terms or terms that are too general will cause you problems. If your terms are too broad or general, the search engine may not process them. Search engines are programmed with various lists of words the designers determined to be so general that a search would turn up hundreds of thousands of references. Check the search engine to see if it has a list of such stopwords . One stopword, for example, is "computers." Some search engines allow you to search stopwords with a specific code (for Google, entering a "+" before the word allows you to search for it).

If your early searches turn up too many references, try searching some relevant ones to find more specific or exact terms. You can start combining these specific terms with NOT (see the section on Boolean operators below) when you see which terms come up in references that are not relevant to your topic. In other words, keep refining your search as you learn more about the terms.

You can also try to make your terms more precise by checking the online catalog of a library. For example, check THOR+ , the Purdue University Library online catalog, and try their subject word search. Or try searching the term in the online databases in the library.

Most search engines now have "Advanced Search" features. These features allow you to use Boolean operators (below) as well as specify other details like date, language, or file type.

Know Boolean operators

Most search engines allow you to combine terms with words (referred to as Boolean operators) such as "and," "or," or "not." Knowing how to use these terms is very important for a successful search. Most search engines will allow you to apply the Boolean operators in an "advanced search" option.

AND is the most useful and most important term. It tells the search engine to find your first word AND your second word or term. AND can, however, cause problems, especially when you use it with phrases or two terms that are each broad in themselves or likely to appear together in other contexts.

For example, if you'd like information about the basketball team Chicago Bulls and type in "Chicago AND Bulls," you will get references to Chicago and to bulls. Since Chicago is the center of a large meat packing industry, many of the references will be about this since it is likely that "Chicago" and "bull" will appear in many of the references relating to the meat-packing industry.

Use OR when a key term may appear in two different ways.

For example, if you want information on sudden infant death syndrome, try "sudden infant death syndrome OR SIDS."

OR is not always a helpful term because you may find too many combinations with OR. For example, if you want information on the American economy and you type in "American OR economy," you will get thousands of references to documents containing the word "American" and thousands of unrelated ones with the word "economy."

NEAR is a term that can only be used on some search engines, and it can be very useful. It tells the search engine to find documents with both words but only when they appear near each other, usually within a few words.

For example, suppose you were looking for information on mobile homes, almost every site has a notice to "click here to return to the home page." Since "home" appears on so many sites, the search engine will report references to sites with the word "mobile" and "click here to return to the home page" since both terms appear on the page. Using NEAR would eliminate that problem.

NOT tells the search engine to find a reference that contains one term but not the other. This is useful when a term refers to multiple concepts.

For example, if you are working on an informative paper on eagles, you may encounter a host of Web sites that discuss the football team the Philadelphia Eagles, instead. To omit the football team from your search results, you could search for "eagles NOT Philadelphia."

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A photo illustration of Databricks, an innovative billion-dollar company with multiple immigrant ... [+] founders. New research finds immigrants play a vital in producing jobs, innovation and new businesses in the United States. (Photo Illustration by Rafael Henrique/SOPA Images/LightRocket via Getty Images)

New research finds immigrants play a vital role in producing jobs, innovation and new businesses in the United States. The findings should interest policymakers and Americans who value expanded employment opportunities, particularly in cutting-edge fields. The research highlights the advantages of immigration overlooked in the current political battles over U.S. border policies.

Immigrant Entrepreneurs

Immigrant entrepreneurs receive little media attention, even though foreign-born business founders influence the nation’s economy. “Immigrants contribute disproportionately to entrepreneurship in many countries, accounting for a quarter of new employer businesses in the U.S.,” according to new research by economists William R. Kerr (Harvard Business School), Saheel A. Chodavadia (University of Michigan), Sari Pekkala Kerr (Wellesley College) and Louis J. Maiden (Harvard Business School). The National Bureau of Economic Research published the study.

The economists detail the impact of immigrant entrepreneurs. “Immigrants are overrepresented as founders of innovative firms and in the high-tech industry,” they write. “In 2022, the four most valuable private, venture-backed U.S. companies (SpaceX, Stripe, Instacart, Databricks) had immigrant founders, along with three of the ten most valuable public companies globally (Alphabet, Nvidia, Tesla).”

A National Foundation for American Policy study found 55% of America’s startup companies valued at $1 billion or more had at least one immigrant founder. Nearly two-thirds (64%) were founded or cofounded by immigrants or the children of immigrants. Approximately one-quarter of the billion-dollar companies in the U.S. had an immigrant founder who came to America as an international student.

An NFAP report concluded immigrants also founded or cofounded nearly two-thirds (65% or 28 of 43) of the top AI companies in the United States. The research examined 43 U.S. companies on the Forbes AI 50, a list of the top startup companies “developing the most promising business applications of artificial intelligence—companies with compelling visions and the resources and technical wherewithal to achieve them.” (I authored both studies.) The economists cited the NFAP studies in their paper in a comparative analysis.

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“Most of the growth of U.S. immigrant entrepreneurship has come through a widespread strengthening of immigrant entrepreneurship across all states rather than a particular boom within a small number of them,” according to the economists.

Immigrant entrepreneurs are innovative. Often innovations become realized by starting a business that allows an entrepreneur to hone and develop a product or service to consumer tastes. The reasons for innovation can be unusual.

When Eric Yuan was a university student in China, he endured 10-hour train rides to visit his girlfriend. That experience encouraged him to envision developing a video conferencing application to communicate with the young woman who would later become his wife. In the United States, he turned the idea into Zoom Video, which is valued today at $19 billion and has over 7,000 employees .

Because U.S. officials denied Yuan a visa, he did not succeed in coming to America until his ninth try. Under U.S. immigration law, there is no startup visa. As a result, before starting a business, most immigrant entrepreneurs in America first obtain permanent residence through family, employment or refugee status.

Kerr, Chodavadia, Kerr and Maiden made a significant finding: A business is “more likely to have a patent and more patents per employee if the owner is an immigrant.” A study by Ufuk Akcigit and Nathan Goldschlag concluded that the immigrant share of inventors in the United States rose from 24% in 2000 to 35% in 2016.

“Firms founded by immigrants are 3.4% to 4.5% more likely to produce new technologies and less likely to use other existing technologies,” according to Kerr, Chodavadia, Kerr and Maiden. “These findings suggest that, even beyond education and field of study, immigrant entrepreneurs tend to be more strongly linked to innovation.”

“The innovativeness of immigrant entrepreneurs, especially in the high-tech sector, suggest links to economic growth, labor adjustment, and agglomeration effects in tech clusters,” according to the economists.

Some aspects of immigrant entrepreneurs remain a mystery. “There is much more to learn about immigrant entrepreneurship,” according to the study. “To begin, we still have a remarkably poor understanding of why immigrants are so entrepreneurial.”

Harvard Business School Professor William Kerr said the traits that lead an individual to immigrate, such as industriousness and a desire for more, have parallels with becoming an entrepreneur.

He agrees that the U.S. economy would benefit if U.S. laws made it easier for entrepreneurs and international students to come to and stay in America. “Yes, full stop,” said Kerr. “Immigrant entrepreneurs are a source of jobs for the U.S. economy and enable a dynamic economy. This is especially true in high-tech and growth-oriented sectors where the U.S. needs to stay at the frontier. It’s what made our nation successful in the past and is necessary for the future.”

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Is Fluoridated Drinking Water Safe for Pregnant Women?

New research suggests a link between prenatal fluoride levels and behavioral issues in children. Experts are divided on the study’s significance.

By Alice Callahan and Christina Caron

A small study published Monday suggested that higher levels of fluoride consumed during the third trimester of pregnancy were associated with a greater risk of behavioral problems in the mothers’ children at 3 years old. The authors of the study, which was funded in part by the National Institutes of Health and the Environmental Protection Agency and published in the journal JAMA Network Open, believe it is the first to examine links between prenatal fluoride exposure and child development among families living in the United States, where fluoride is often added to community water supplies to prevent dental cavities.

The study’s authors and some outside researchers said that the findings should prompt policymakers to evaluate the safety of fluoride consumption during pregnancy.

“I think it’s a warning sign,” said Dr. Beate Ritz, an environmental epidemiologist at the U.C.L.A. Fielding School of Public Health.

But other experts cautioned that the study had several important limitations that made it difficult to assess the potential effects of fluoride consumption during pregnancy.

“There is nothing about this study that alarms me or would make me recommend that pregnant women stop drinking tap water,” said Dr. Patricia Braun, a professor of pediatrics at the University of Colorado School of Medicine and a spokesperson for the American Academy of Pediatrics.

The Background

Fluoride strengthens tooth enamel, and research suggests that drinking water with added fluoride can reduce cavities by up to 25 percent . Many communities in the United States have added fluoride to their water for this reason since the 1940s, a practice widely celebrated as a major public health achievement . In 2020, 63 percent of people in the United States lived in areas with at least 0.7 milligrams per liter of fluoride in the water — considered optimal for cavity prevention — though some areas have levels that are higher , in part because of naturally high fluoride in the groundwater.

In the last few years, several studies from Mexico and Canada have suggested that fluoride exposure during pregnancy is linked to slightly lower scores on intelligence tests and other measures of cognitive function in children.

But recent studies from Spain and Denmark have found no such link.

There is a “contentious debate” about water fluoridation, acknowledged Ashley Malin, an assistant professor of epidemiology in the College of Public Health and Health Professions at the University of Florida and the lead author of the new study. The issue is currently the subject of a lawsuit filed by the nonprofit Food and Water Watch and other groups against the Environmental Protection Agency. The nonprofit claims that water fluoridation poses a risk to children’s health.

The Research

The study looked at a group of 229 predominantly low-income Hispanic pregnant women in Los Angeles who were already being followed in other research. Most of the women lived in areas with fluoridated water. The researchers measured the fluoride levels in their urine in a single test during the third trimester. Then, when their children were 3 years old, the mothers filled out the Preschool Child Behavior Checklist , a measure used to detect emotional, behavioral and social problems.

Overall, 14 percent of the children had a total score in the “borderline clinical” or “clinical” range, meaning that a doctor may want to watch or evaluate them, or provide additional support, Dr. Malin said. And on average, higher fluoride levels in the mothers’ urine were correlated with a greater risk of behavioral problems in the children. The researchers found that women with urine fluoride levels at the 75th percentile were 83 percent more likely to have children with borderline or clinically significant behavioral problems than women with levels at the 25th percentile.

The main problems reported by the mothers were emotional reactivity, which is the tendency to overreact; somatic complaints, such as headaches and stomachaches; anxiety; and symptoms linked to autism (though those symptoms alone would not be enough for an autism diagnosis).

The researchers did not find an association with other behavioral symptoms like aggression or issues with concentration.

The findings are important and add to evidence suggesting prenatal fluoride consumption may affect the developing brain, said Joseph Braun, a professor of epidemiology and the director of the Center for Children’s Environmental Health at Brown University, who was not involved in the research. That said, the increases in behavioral scores were relatively small — about two points on a scale from 28 to 100 for overall behavioral problems. It’s hard to say whether such a difference might be noticeable in an individual child, he said.

But given how widespread water fluoridation is, he added, even minor behavioral changes in individual children could have a meaningful impact on the overall population.

The Limitations

The study was relatively small and didn’t include a diverse group of women. It didn’t account for many factors that can affect child development, including genetics , maternal nutrition, home environment and community support, several experts not involved in the study said.

The data would also have been stronger if the researchers had measured fluoride in urine samples from several points of time during pregnancy and collected information on tap water, bottled water and tea consumption to better understand how each contributed to the women’s fluoride levels, experts said. Black and green teas can contain high levels of fluoride.

The Preschool Child Behavior Checklist that was used to evaluate the 3-year-olds is considered a reliable measure of child behavior. But it did not take into account the fact that symptoms can change in frequency and intensity during early childhood, said Catherine Lord, an expert on autism and related disorders at the University of California, Los Angeles medical school.

Dr. Lord, who was not involved in the fluoride research, added that the checklist is not considered a reliable way to diagnose autism.

It would also be helpful to follow the children to see if the problematic behaviors persisted beyond age 3, said Melissa Melough, an assistant professor of nutrition sciences at the University of Delaware, who was not involved in the research.

What’s Next

While the experts agreed that more robust research was needed to untangle the potential effects of prenatal fluoride exposure, they had differing opinions about the study’s bottom line.

Dr. Malin said that, based on her findings and the evidence from previous studies, it might be a good idea for women to limit fluoride intake during pregnancy, a view that was echoed by Dr. Ritz and others.

“For me, the takeaway is: Protect pregnancy,” said Marcela Tamayo-Ortiz, an environmental epidemiologist at the Columbia University Mailman School of Public Health who has studied prenatal exposures for more than two decades.

But the American Dental Association said in a statement that the organization stands by its recommendation to “brush twice a day with fluoride toothpaste and drink optimally fluoridated water.”

And Dr. Nathaniel DeNicola, an OB-GYN and environmental health expert in Orange County, Calif., said he wouldn’t advise his pregnant patients to avoid fluoridated water based on the study, because “it’s not conclusive.”

Dr. Melough said she didn’t think women should be alarmed by the findings. But, she added, while it’s clear that fluoride helps to reduce cavities, it’s possible that adding it to the water “could have some unintended consequences,” and policymakers should continually evaluate the practice as new science emerges.

You can find out what the fluoride levels are in your local water by contacting your water utility or checking the C.D.C.’s My Water’s Fluoride website . If you want to reduce your fluoride consumption, experts said, limit how much black or green tea you drink. You can also purchase certain water filters that remove some fluoride. There’s no reason to stop brushing your teeth with fluoride toothpaste — just don’t swallow it.

Alice Callahan is a Times reporter covering nutrition and health. She has a Ph.D. in nutrition from the University of California, Davis. More about Alice Callahan

Christina Caron is a Times reporter covering mental health. More about Christina Caron

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When Online Content Disappears

38% of webpages that existed in 2013 are no longer accessible a decade later, table of contents.

• Webpages from the last decade
• Links on government websites
• Links on news websites
• Reference links on Wikipedia
• Posts on Twitter
• Acknowledgments
• Collection and analysis of Twitter data
• Data collection for World Wide Web websites, government websites and news websites
• Data collection for Wikipedia source links
• Evaluating the status of pages and links
• Definition of links

Pew Research Center conducted the analysis to examine how often online content that once existed becomes inaccessible. One part of the study looks at a representative sample of webpages that existed over the past decade to see how many are still accessible today. For this analysis, we collected a sample of pages from the Common Crawl web repository for each year from 2013 to 2023. We then tried to access those pages to see how many still exist.

A second part of the study looks at the links on existing webpages to see how many of those links are still functional. We did this by collecting a large sample of pages from government websites, news websites and the online encyclopedia Wikipedia .

We identified relevant news domains using data from the audience metrics company comScore and relevant government domains (at multiple levels of government) using data from get.gov , the official administrator for the .gov domain. We collected the news and government pages via Common Crawl and the Wikipedia pages from an archive maintained by the Wikimedia Foundation . For each collection, we identified the links on those pages and followed them to their destination to see what share of those links point to sites that are no longer accessible.

A third part of the study looks at how often individual posts on social media sites are deleted or otherwise removed from public view. We did this by collecting a large sample of public tweets on the social media platform X (then known as Twitter) in real time using the Twitter Streaming API. We then tracked the status of those tweets for a period of three months using the Twitter Search API to monitor how many were still publicly available. Refer to the report methodology for more details.

The internet is an unimaginably vast repository of modern life, with hundreds of billions of indexed webpages. But even as users across the world rely on the web to access books, images, news articles and other resources, this content sometimes disappears from view.

A new Pew Research Center analysis shows just how fleeting online content actually is:

• A quarter of all webpages that existed at one point between 2013 and 2023 are no longer accessible, as of October 2023. In most cases, this is because an individual page was deleted or removed on an otherwise functional website.

• For older content, this trend is even starker. Some 38% of webpages that existed in 2013 are not available today, compared with 8% of pages that existed in 2023.

This “digital decay” occurs in many different online spaces. We examined the links that appear on government and news websites, as well as in the “References” section of Wikipedia pages as of spring 2023. This analysis found that:

• 23% of news webpages contain at least one broken link, as do 21% of webpages from government sites. News sites with a high level of site traffic and those with less are about equally likely to contain broken links. Local-level government webpages (those belonging to city governments) are especially likely to have broken links.
• 54% of Wikipedia pages contain at least one link in their “References” section that points to a page that no longer exists.

To see how digital decay plays out on social media, we also collected a real-time sample of tweets during spring 2023 on the social media platform X (then known as Twitter) and followed them for three months. We found that:

• Nearly one-in-five tweets are no longer publicly visible on the site just months after being posted. In 60% of these cases, the account that originally posted the tweet was made private, suspended or deleted entirely. In the other 40%, the account holder deleted the individual tweet, but the account itself still existed.
• Certain types of tweets tend to go away more often than others. More than 40% of tweets written in Turkish or Arabic are no longer visible on the site within three months of being posted. And tweets from accounts with the default profile settings are especially likely to disappear from public view.

How this report defines inaccessible links and webpages

There are many ways of defining whether something on the internet that used to exist is now inaccessible to people trying to reach it today. For instance, “inaccessible” could mean that:

• The page no longer exists on its host server, or the host server itself no longer exists. Someone visiting this type of page would typically receive a variation on the “404 Not Found” server error instead of the content they were looking for.
• The page address exists but its content has been changed – sometimes dramatically – from what it was originally.
• The page exists but certain users – such as those with blindness or other visual impairments – might find it difficult or impossible to read.

For this report, we focused on the first of these: pages that no longer exist. The other definitions of accessibility are beyond the scope of this research.

Our approach is a straightforward way of measuring whether something online is accessible or not. But even so, there is some ambiguity.

First, there are dozens of status codes indicating a problem that a user might encounter when they try to access a page. Not all of them definitively indicate whether the page is permanently defunct or just temporarily unavailable. Second, for security reasons, many sites actively try to prevent the sort of automated data collection that we used to test our full list of links.

For these reasons, we used the most conservative estimate possible for deciding whether a site was actually accessible or not. We counted pages as inaccessible only if they returned one of nine error codes that definitively indicate that the page and/or its host server no longer exist or have become nonfunctional – regardless of how they are being accessed, and by whom. The full list of error codes that we included in our definition are in the methodology .

Here are some of the findings from our analysis of digital decay in various online spaces.

To conduct this part of our analysis, we collected a random sample of just under 1 million webpages from the archives of Common Crawl , an internet archive service that periodically collects snapshots of the internet as it exists at different points in time. We sampled pages collected by Common Crawl each year from 2013 through 2023 (approximately 90,000 pages per year) and checked to see if those pages still exist today.

We found that 25% of all the pages we collected from 2013 through 2023 were no longer accessible as of October 2023. This figure is the sum of two different types of broken pages: 16% of pages are individually inaccessible but come from an otherwise functional root-level domain; the other 9% are inaccessible because their entire root domain is no longer functional.

Not surprisingly, the older snapshots in our collection had the largest share of inaccessible links. Of the pages collected from the 2013 snapshot, 38% were no longer accessible in 2023. But even for pages collected in the 2021 snapshot, about one-in-five were no longer accessible just two years later.

We sampled around 500,000 pages from government websites using the Common Crawl March/April 2023 snapshot of the internet, including a mix of different levels of government (federal, state, local and others). We found every link on each page and followed a random selection of those links to their destination to see if the pages they refer to still exist.

Across the government websites we sampled, there were 42 million links. The vast majority of those links (86%) were internal, meaning they link to a different page on the same website. An explainer resource on the IRS website that links to other documents or forms on the IRS site would be an example of an internal link.

Around three-quarters of government webpages we sampled contained at least one on-page link. The typical (median) page contains 50 links, but many pages contain far more. A page in the 90th percentile contains 190 links, and a page in the 99th percentile (that is, the top 1% of pages by number of links) has 740 links.

Other facts about government webpage links:

• The vast majority go to secure HTTP pages (and have a URL starting with “https://”).
• 6% go to a static file, like a PDF document.
• 16% now redirect to a different URL than the one they originally pointed to.

When we followed these links, we found that 6% point to pages that are no longer accessible. Similar shares of internal and external links are no longer functional.

Overall, 21% of all the government webpages we examined contained at least one broken link. Across every level of government we looked at, there were broken links on at least 14% of pages; city government pages had the highest rates of broken links.

For this analysis, we sampled 500,000 pages from 2,063 websites classified as “News/Information” by the audience metrics firm comScore. The pages were collected from the Common Crawl March/April 2023 snapshot of the internet.

Across the news sites sampled, this collection contained more than 14 million links pointing to an outside website. 1 Some 94% of these pages contain at least one external-facing link. The median page contains 20 links, and pages in the top 10% by link count have 56 links.

Like government websites, the vast majority of these links go to secure HTTP pages (those with a URL beginning with “https://”). Around 12% of links on these news sites point to a static file, like a PDF document. And 32% of links on news sites redirected to a different URL than the one they originally pointed to – slightly less than the 39% of external links on government sites that redirect.

When we tracked these links to their destination, we found that 5% of all links on news site pages are no longer accessible. And 23% of all the pages we sampled contained at least one broken link.

Broken links are about as prevalent on the most-trafficked news websites as they are on the least-trafficked sites. Some 25% of pages on news websites in the top 20% by site traffic have at least one broken link. That is nearly identical to the 26% of sites in the bottom 20% by site traffic.

For this analysis, we collected a random sample of 50,000 English-language Wikipedia pages and examined the links in their “References” section. The vast majority of these pages (82%) contain at least one reference link – that is, one that directs the reader to a webpage other than Wikipedia itself.

In total, there are just over 1 million reference links across all the pages we collected. The typical page has four reference links.

The analysis indicates that 11% of all references linked on Wikipedia are no longer accessible. On about 2% of source pages containing reference links, every link on the page was broken or otherwise inaccessible, while another 53% of pages contained at least one broken link.

For this analysis, we collected nearly 5 million tweets posted from March 8 to April 27, 2023, on the social media platform X, which at the time was known as Twitter. We did this using Twitter’s Streaming API, collecting 3,000 public tweets every 30 minutes in real time. This provided us with a representative sample of all tweets posted on the platform during that period. We monitored those tweets until June 15, 2023, and checked each day to see if they were still available on the site or not.

At the end of the observation period, we found that 18% of the tweets from our initial collection window were no longer publicly visible on the site . In a majority of cases, this was because the account that originally posted the tweet was made private, suspended or deleted entirely. For the remaining tweets, the account that posted the tweet was still visible on the site, but the individual tweet had been deleted.

Which tweets tend to disappear?

Tweets were especially likely to be deleted or removed over the course of our collection period if they were:

• Written in certain languages. Nearly half of all the Turkish-language tweets we collected – and a slightly smaller share of those written in Arabic – were no longer available at the end of the tracking period.
• Posted by accounts using the site’s default profile settings. More than half of tweets from accounts using the default profile image were no longer available at the end of the tracking period, as were more than a third from accounts with a default bio field. Tweets from these accounts tend to disappear because the entire account has been deleted or made private, as opposed to the individual tweet being deleted.
• Posted by unverified accounts.

We also found that removed or deleted tweets tended to come from newer accounts with relatively few followers and modest activityon the site. On average, tweets that were no longer visible on the site were posted by accounts around eight months younger than those whose tweets stayed on the site.

And when we analyzed the types of tweets that were no longer available, we found that retweets, quote tweets and original tweets did not differ much from the overall average. But replies were relatively unlikely to be removed – just 12% of replies were inaccessible at the end of our monitoring period.

Most tweets that are removed from the site tend to disappear soon after being posted. In addition to looking at how many tweets from our collection were still available at the end of our tracking period, we conducted a survival analysis to see how long these tweets tended to remain available. We found that:

• 1% of tweets are removed within one hour
• 3% within a day
• 10% within a week
• 15% within a month

Put another way: Half of tweets that are eventually removed from the platform are unavailable within the first six days of being posted. And 90% of these tweets are unavailable within 46 days.

Tweets don’t always disappear forever, though. Some 6% of the tweets we collected disappeared and then became available again at a later point. This could be due to an account going private and then returning to public status, or to the account being suspended and later reinstated. Of those “reappeared” tweets, the vast majority (90%) were still accessible on Twitter at the end of the monitoring period.

• For our analysis of news sites, we did not collect or check the functionality of internal-facing on-page links – those that point to another page on the same root domain. ↩

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Sites with radioactive material more vulnerable as climate change increases wildfire, flood risks

Climate climate change increasingly threatens research laboratories, weapons sites and power plants across the nation that handle or are contaminated with radioactive material or perform critical energy and defense research

Climate change increasingly threatens some of the nation’s most sensitive sites, including research laboratories, military facilities and power plants with radioactive material.

Extreme heat and drought, longer fire seasons with larger, more intense blazes and supercharged rainstorms that can lead to catastrophic flooding are forcing a reckoning that environmentalists and experts say is long overdue.

Many sites are contaminated or warehouse decades of radioactive waste, while some perform critical energy and defense research and manufacturing that could be crippled by increasingly unpredictable extreme weather.

Among them: The 40-square-mile Los Alamos National Laboratory in New Mexico, where a 2000 wildfire burned to within a half mile (0.8 kilometers) of a radioactive waste site. The Santa Susana Field Laboratory in Southern California, where a 2018 wildfire burned 80% of the site, narrowly missing an area contaminated by a 1959 partial nuclear meltdown. And the plutonium-contaminated Hanford nuclear site in Washington, where the U.S. manufactured atomic bombs.

In February, wildfires came within 3 miles (5 kilometers) of the Pantex Plant in Texas, which assembles and disassembles nuclear weapons and stores thousands of plutonium pits — hollow spheres that trigger nuclear warheads and bombs.

Fire didn’t reach the site, and officials said plutonium pits — in fire-resistant drums and shelters — likely would not have been affected. But the size and speed of the fires, urgent efforts to dig firebreaks and the decision to send workers home underscore what’s at stake.

The Texas fire season often starts in February, but farther west it has yet to ramp up.

“I think we’re still early in recognizing climate change and ... how to deal with these extreme weather events,” said Paul Walker, program director at Green Cross International and a former House Armed Services Committee staffer. “What might have been safe 25 years ago probably is no longer safe.”

That realization has begun to change how the government addresses threats.

The Department of Energy in 2022 required sites to assess climate risks to “mission-critical functions and operations,” and plan for them. It cited wildfires at two national laboratories and a 2021 freeze that damaged “critical facilities” at Pantex.

Yet the agency does not consider future climate risks when authorizing new sites or projects, or in periodic environmental assessments. It only considers how sites themselves might affect climate change, which critics call short-sighted and potentially dangerous.

Likewise, the Nuclear Regulatory Commission considers only historical climate data in licensing decisions and nuclear plant oversight, according to a General Accounting Office study in April that recommended NRC “fully consider potential climate change effects.” The GAO found that 60 of 75 U.S. plants were in areas with high flood hazard and 16 with high wildfire potential.

“We’re acting like ... (what’s) happening now is what we can expect to happen in 50 years,” said Caroline Reiser, a climate and energy attorney at the Natural Resources Defense Council. “The reality of what our climate is doing has shifted dramatically, and we need to shift our planning.”

The National Nuclear Security Administration’s environmental safety and health division, which oversees active DOE sites, will develop “crucial” methodologies to address climate risks in permitting and site assessments, said John Weckerle, the division’s director of environmental regulatory affairs.

“We all know the climate is changing. Everybody’s thinking about, what effect are we having on the climate?” Weckerle said. “Now we need to flip that on its head and say, ‘OK ... but what do we think is going to happen as a result of climate on a particular site?’”

Experts say risks vary. Most plutonium and other radioactive material is in concrete or steel structures or underground. And many sites are remote, where public risk likely would be minimal.

Still, potential threats have arisen.

In 2000, a wildfire burned one-third of the 580-square-mile (1,502-square-kilometer) Hanford site, which produced plutonium for the U.S. atomic weapons program and is considered the nation’s most radioactive place.

Air monitoring detected plutonium in nearby populated areas at levels higher than background, but only for one day at levels not considered hazardous, according to a Washington health department report.

The state said the plutonium likely was from surface soil blown by wind during and after the fire.

A 2018 fire in California started at the Santa Susana Field Laboratory, a former nuclear research and rocket-engine testing site, and burned within several hundred feet of contaminated buildings and soil, and near where a nuclear reactor core partially melted down 65 years ago.

The state’s Department of Toxic Substances Control muti-agency sampling found no off-site radioactive or other hazardous material from the fire. But an outside study found radioactive microparticles in ash beyond of the lab boundary.

The state ordered 18 buildings demolished, citing “substantial endangerment to people and the environment,” because future fires could release radioactive and hazardous substances.

It ordered cleanup of old burn pits contaminated with radioactive materials, fearing fire or floods could damage tarps covering them.

A 2000 wildfire burned 7,500 acres (3,035 hectares) at the Los Alamos National Laboratory, coming within a half-mile (0.8 kilometers) of more than 24,000 above-ground containers of mostly plutonium-contaminated waste.

Most containers have since been shipped to offsite storage. Remaining radioactive material — including from the Manhattan Project — now is underground or in containers beneath fire-retardant fabric-and-steel domes.

The lab’s fire preparedness includes thinning forests, said Rich Nieto, manager of its wildland fire program. “What used to be a three-month (fire) season, sometimes will be a six-month season,” he said.

Fire isn’t the only threat. Intense rainstorms can wash away contaminated sediment. Floods and extreme cold have forced the shutdown of several DOE sites in recent years.

In 2010, Pantex was inundated with rain that affected operations for almost a month and flooded a plutonium storage area. In 2021, it was shut down for a week because of extreme cold that officials said led to “freeze-related failures” at 10 nuclear facilities there.

Pantex has since adopted freeze-protection measures, upgraded fire and electrical systems and installed backup generators.

Other DOE sites are looking at their own needs, the nuclear security agency’s Weckerle said.

“We live in a time of increased risk,” he said. “That’s just the heart of it (and) ... a lot of that does have to do with climate change.”

The Associated Press receives support for nuclear security coverage from the Carnegie Corporation of New York and Outrider Foundation . The AP is solely responsible for all content.

Extremist communities continue to rely on YouTube for hosting, but most videos are viewed off-site, Northeastern research finds

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It’s easy to fall down the rabbit hole of the online video-sharing platform YouTube. After all, is there really a limit to cute pet videos?

But what about the platform’s more sinister side? After the 2016 U.S. presidential election, YouTube was so criticized for radicalizing users by recommending increasingly extremist and fringe content that it changed its recommendation algorithm .

Research four years later by Northeastern University computer scientist Christo Wilson found that — while extremist content remained on YouTube — subscriptions and external referrals drove disaffected users to extremist content rather than the recommendation algorithm. 

“We didn’t see this kind of ‘rabbit-holing effect,’” says Wilson, an associate professor at Khoury College of Computer Sciences at Northeastern. “There was in fact a lot of problematic content that was still on YouTube and still had a fairly significant audience of people who were watching it. It’s just that they haven’t been radicalized on YouTube itself.”

So if not on YouTube, where was this audience being radicalized? 

In new research presented at a conference on Tuesday , Wilson finds that extremist communities continue to rely on YouTube for video hosting — it’s just that off-site is where the “rabbit-holing” begins.

“If you’re already a political partisan and you’re going to websites with a particular leaning, that’s then leading you to YouTube channels and videos with the same kind of lean,” Wilson says. “If you started in a place where you’re being exposed to bad stuff, you end up in a place where you’re being exposed to more bad stuff.”

YouTube is an online video-sharing platform owned by Google. Following criticism for its role in hosting and elevating fringe conspiracy content, particularly through its recommendation algorithm, the platform changed that algorithm in 2019 . 

But the extremist content never fully disappeared. 

Much of it migrated.

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“YouTube is not just YouTube itself — you can embed the videos into any website,” Wilson says. “This is the first study where we’re looking at all this stuff that happens off-platform.” 

Wilson looked at over 1,000 U.S. residents from three cohorts: demographically representative users, heavy YouTube users, and users with high racial resentment. He analyzed all YouTube videos encountered by the users over a period of six months. All users accessed the web via desktop, rather than through mobile devices. 

The research resulted in several interesting conclusions. 

Wilson found that users saw more YouTube videos on sites other than YouTube than on the platform’s website itself.

He also found that politically right-leaning websites tend to embed more videos from “problematic” YouTube channels than centrist or left-leaning websites. Channels were considered problematic if they were classified as either alternative or extremist, using grades assigned by professional fact checkers or other academics. 

Wilson says an alternative channel would be, for example, Steven Crowder — a personality who interviews both mainstream scientists and vaccine deniers and is “sort of intellectually open.”  Extremist channels, Wilson said, would be “openly hateful” — something like former Ku Klux Klan Grand Wizard David Duke’s old YouTube channel.

Most notably, users exposed to off-platform videos from problematic channels are significantly more inclined to browse toward on-platform videos from problematic channels. 

“Your off-platform activity very quickly becomes on-platform activity,” Wilson says.

So, what can YouTube do? After all, Wilson admits the platform can’t control what people do when on other sites.

Wilson recommends stronger content-moderation policies. 

“YouTube can tell where videos are being embedded off platform,” Wilson notes. “If they see a particular channel being embedded in a website that is a known purveyor of misinformation, that channel should probably be scrutinized.”

Plus, Wilson notes that YouTube still hosts the videos, even if they appear on other sites. 

“They are aiding and abetting these fringe communities out there on the web by hosting videos for them,” Wilson says. “If they had stronger content-moderation policies, that would definitely help address this.”

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Pokémon Go ‘Ultra Space Wonders’ event Timed Research and rewards

Stakataka and Blacephalon make their debut in Pokémon Go

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Share All sharing options for: Pokémon Go ‘Ultra Space Wonders’ event Timed Research and rewards

Pokémon Go is hosting an “ Ultra Space Wonders ” event to wrap up the “World of Wonders” season, which will end in June. The event runs from May 23-28 and it features the debut of the Ultra Beasts Stakataka and Blacephalon .

The event also boosts the spawn of poison-type Pokémon, adds the last leg of the “World of Wonders” Special Research , and rewards doubled XP for winning raids against Ultra Beasts. Mareanie is also available shiny for the first time ever .

Below, we list out all the perks of the Pokémon Go “Ultra Space Wonders” event, including the paid Timed Research, Collection Challenge, and event Field Research.

This is a paid research for $5 . Once you buy it, you’ll only have until May 28 at 8 p.m. in your local time to complete it.

Is the ‘Ultra Space Wonders’ paid research ticket worth it?

The value of the “Ultra Space Wonders” paid research ticket is technically worth it, but I would only buy this if you are really hunting for shiny Mareanie or if you want the Naganadel Wings cosmetic.

Step 1 of 1

• Catch 5 poison-type Pokémon (Mareanie encounter)
• Earn 2,000 Stardust (10 Revives)
• Catch 10 poison-type Pokémon (Mareanie encounter)
• Earn 4,000 Stardust (10 Hyper Potions)
• Catch 15 poison-type Pokémon (Mareanie encounter)
• Earn 6,000 Stardust (15 Poké Balls)
• Catch 20 poison-type Pokémon (Mareanie encounter)
• Earn 8,000 Stardust (15 Great Balls)
• Catch 25 poison-type Pokémon (Mareanie encounter)
• Earn 10,000 Stardust (15 Ultra Balls)
• Catch 30 poison-type Pokémon (Mareanie encounter)
• Win a raid (1 Lucky Egg)

Rewards : 4 Premium Battle Passes, Mareanie encounter, Naganadel Wings cosmetic

Pokémon Go ‘Ultra Space Wonders’ event Collection Challenge

There are a few Collection Challenges to complete that involve you catching and evolving specific Pokémon.

Ultra Space Wonders Collection Challenge: Catch

• Catch a Tentacool
• Catch a Mareanie
• Catch a Skrelp

Rewards : 1,000 XP, Mareanie encounter

Ultra Space Wonders Collection Challenge: Research

• Catch a Nidoran-F
• Catch a Nidoran-M
• Catch a Trubbish

Rewards : 5,000 XP, Mareanie encounter

Ultra Space Wonders Collection Challenge: Raid

• Catch a Bagon
• Catch a Deino
• Catch a Druddigon

Rewards : 10,000 XP, Mareanie encounter

Pokémon Go ‘Ultra Space Wonders’ event Field Research and rewards

Spinning a PokéStop during the event period may yield one of these tasks:

• Catch 10 Pokémon (Nidoran-F or Nidoran-M encounter)
• Power up Pokémon 5 times (Trubbish encounter)
• Power up Pokémon 10 times (Mareanie encounter)
• Win a raid (Goomy encounter)
• Win 3 raids (Jangmo-o encounter)

Pokémon Go ‘Ultra Space Wonders’ event boosted spawns

These Pokémon will spawn more frequently during the event period:

Pokémon Go ‘Ultra Space Wonders’ event raid targets

The following Pokémon will be in raids during the event:

• Paldean Wooper (1-star)
• Hisuian Qwilfish (1-star)
• Hisuian Sneasel (1-star)
• Bagon (1-star)
• Deino (1-star)
• Galarian Weezing (3-star)
• Druddigon (3-star)
• Turtonator (3-star)
• Stakataka (5-star, eastern hemisphere only)
• Blacephalon (5-star, western hemisphere only)
• Mega Pidgeot (Mega)
• Pokémon Go guides
• “Rediscover Kanto” Special Research
• How to find Wiglett
• Ditto disguises

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