The Younger Dryas impact hypothesis (YDIH) is an unpopular idea that suggests a sudden flood of freshwater into the ocean caused changes in ocean currents and led to a cold period called the Younger Dryas, around 12,900 years ago. Scientists have rejected this theory for many years because there is no proof of an impact event. Studies by experts in geology, astronomy, and other fields have shown the idea is inconsistent, ignores important information, and sometimes contradicts basic scientific laws. Many scientists believe the debate over YDIH is unusual because its main supporters claim that most scientists disagree with it unfairly and suggest a cover-up by the scientific community.
The YDIH idea became more widely known through documentaries on ancient history and books by author Graham Hancock. Hancock’s theories, which include YDIH, were discussed in a 2022 episode of the Netflix series Ancient Apocalypse. In 2024, the New York Times described YDIH as "The Comet Strike Theory That Just Won't Die," noting that it continues to attract public interest despite being discredited. Supporters of YDIH often describe scientists who oppose it as part of a secretive, powerful group. The theory especially appeals to people who believe in conspiracy theories and distrust scientific experts.
Proponents of YDIH created a non-profit organization called Comet Research Group Inc. to study impact events, such as the one YDIH claims caused the Younger Dryas. One of CRG’s most famous studies, published in 2021, examined an archaeological site called Tell el-Hammam and claimed it was the ruins of the Biblical city of Sodom, destroyed by God as described in the Book of Genesis. However, the study was later found to contain altered data and was retracted in 2025 by the journal Scientific Reports. This was the second study by CRG members to be retracted from the same journal.
History
The idea that a comet hit North America at the end of the last ice age was first proposed in the 19th century by Ignatius Donnelly, a politician and believer in secret plans. He wrote about this in his 1883 book Ragnarok, suggesting that the comet caused clay and gravel deposits across North America and led to the destruction of a fictional lost civilization called Atlantis.
The Younger Dryas Impact Hypothesis (YDIH) is an uncommon idea that tries to explain the sudden cooling at the end of the last ice age, known as the Younger Dryas (YD) period. It was first presented in 2007 by nuclear physicist Richard Firestone and other scientists in a scientific journal called Proceedings of the National Academy of Sciences of the United States of America. Their paper, titled Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling, claimed that an impact from space caused the cooling. Before this, Firestone and an archaeologist named William Topping wrote about the idea in 2001 in a newsletter called Mammoth Trumpet.
Firestone and his team expanded on the idea in their 2006 book The Cycle of Cosmic Catastrophes, though one of the book’s co-authors said the book was not meant to be scientific. They argued that the impact broke an ice dam holding a large lake called Lake Agassiz, causing it to flood the Atlantic Ocean. This flood disrupted ocean currents and caused sudden cooling. They also claimed the impact destabilized a large ice sheet, leading to wildfires, the extinction of large animals, and the disappearance of a group of ancient people called the Clovis culture. However, genetic evidence suggests the Clovis people moved south and adapted to changes in their environment. Some scientists, like Vance T. Holliday, argue that there is no clear break in the archaeological record and that the Clovis people moved frequently, with tools from a later culture called Folsom appearing at the same time as the disappearance of tools used for hunting large animals.
The widely accepted explanation for the sudden influx of freshwater into the ocean is melting ice from retreating ice sheets, as described in a 1989 study in the journal Nature by geochemist Wallace Smith Broecker and others. James Kennett, a paleoceanographer, was a co-author of both the 1989 study and the 2007 YDIH paper.
The 2007 YDIH paper faced criticism from scientists in many fields, including geologists, astronomers, archaeologists, and paleoecologists. Supporters of YDIH claim that the opposition comes from a small group of critics who spread unfounded claims about a cover-up. However, scientists like Nicholas Pinter found no evidence of the impact, could not reproduce key findings, and suggested other natural explanations for materials found in the Earth. In 2011, Pinter and others tested YDIH and concluded that the hypothesis was rejected because the 2007 study’s results could not be reproduced and were likely due to normal geological processes.
A major claim of YDIH was the discovery of high levels of nanodiamonds in carbon-rich spherules, which were thought to form from a comet impact. Scientists speculated that an air burst from the comet could create these diamonds. However, a 2010 study by physicist Tyrone Daulton found no nanodiamonds, and a study by geophysicist Jay Melosh showed that an air burst could not create enough pressure to form them. A 2010 study by paleobotanist Andrew C. Scott found that the materials were actually from fungi or insect waste. Additional studies in 2016 confirmed these findings.
Supporters of YDIH also pointed to a spike in platinum found in a Greenland ice core from around 13,000 years ago and the 2015 discovery of the Hiawatha impact structure. However, in 2018, Melosh argued that the Hiawatha impact was unlikely to have caused the YD cooling because it was too small and too old. A 2025 study by volcanologist Charlotte E. Green found that the platinum spike occurred 45 years after the start of the YD period and lasted only 14 years, which does not support a sudden event. Instead, Green concluded the platinum came from volcanic eruptions in Iceland.
Other evidence cited for YDIH includes black mats, or layers of soil rich in organic material, and magnetic spherules found at archaeological sites. A 2009 study by Jennifer Marlon found no spike in charcoal or pollen data that matched the YD cooling. West, a co-author of the 2007 paper,
Public interest
YDIH became widely known because it was included in documentaries shown on the National Geographic Channel, History Channel, and the PBS program NOVA. In 2024, The New York Times (NYT) described YDIH as "The Comet Strike Theory That Just Won't Die" in an article that explained its history and why people remain interested in it. People often believe YDIH even though scientists disagree with it because of a mental process called "epistemic vigilance," which helps individuals tell real information from fake information. Psychologists have found that when people hear conflicting information from experts, they often choose the side that matches their existing beliefs or their political or cultural background. Psychologist Spencer Mermelstein explained that YDIH may be appealing because it offers a simple explanation that connects with people's understanding of Earth's history, suggesting "one big cause, one big outcome."
In his 2015 book Magicians of the Gods, Graham Hancock claimed that the Younger Dryas comet caused widespread destruction on Earth during a time cycle and was linked to the Noahide flood myth. He compared this myth to flood stories from other cultures, suggesting the idea was common worldwide. These claims were criticized as incorrect by reviewers such as Jason Colavito, Michael Shermer, and Marc J. Defant. Hancock expanded his ideas in his 2019 book America Before: The Key to Earth's Lost Civilization, where he argued that the Younger Dryas catastrophe erased evidence of a complex Ice Age civilization in North America. The Society for American Archaeology (SAA) issued an open letter stating that Hancock's ideas about such a civilization promoted "dangerous racist thinking."
The SAA's letter urged Netflix to reclassify Hancock's 2022 docuseries, Ancient Apocalypse, as science fiction. The series focused on Hancock's belief in a lost civilization, including the fictional Atlantis, and discussed YDIH in Episode 8. Impact physicist Mark Boslough wrote in Skeptic magazine that many parts of the series are pseudoscience. Articles in The Guardian, Slate, The Nation, and other left-leaning publications criticized the series, while conservative outlets praised it and labeled criticism as "left-wing propaganda." Political activist Tucker Carlson referred to the SAA as an "elitist, closed-minded cabal" connected to the "collapse of the American idea."
The YouTube community has increased YDIH's visibility through many videos about the topic. Similar identity-based divisions appear on YouTube as they do in mainstream media, with YouTubers who support YDIH often portraying skeptics as part of a "scientific cabal" or victims of groupthink. YDIH continues to gain popularity despite scientific disagreement because it captures the public's imagination and connects with personal and group identity rather than scientific research. It especially appeals to people who believe in conspiracy theories that reject scientific expertise.