The Younger Dryas impact hypothesis (YDIH) is an unpopular and disputed 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. Many scientists in fields like geology and astronomy have rejected YDIH for decades because there is no strong evidence to support it. Studies have shown that the idea contradicts itself, ignores important information, and sometimes goes against scientific laws. Scientists who study YDIH are often criticized for claiming that most experts disagree with them and that mainstream science is hiding the truth.
YDIH became well-known through documentaries on ancient history and books by writer Graham Hancock, who promotes ideas not based on real science. A 2022 episode of the Netflix series Ancient Apocalypse discussed Hancock’s claims about YDIH. 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 even though it is not supported by science. Some supporters of YDIH call scientists who oppose it "villains" part of secretive groups. People who believe in conspiracy theories often support YDIH, especially those who already distrust scientific experts.
Supporters of YDIH created a non-profit group called Comet Research Group Inc. (CRG) to study impacts like the one proposed by YDIH. One of CRG’s most famous studies, published in 2021, claimed that an ancient site called Tell el-Hammam was the ruins of the Biblical city of Sodom, destroyed by God as described in the Book of Genesis. However, this study was later found to have false data and was retracted in 2025 by the journal Scientific Reports.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 suggested in the 19th century by Ignatius Donnelly, a politician and writer who also believed in secret plans. In his 1883 book Ragnarok, he claimed that a comet impact caused clay and gravel found across North America and led to the destruction of a fictional lost civilization called Atlantis.
The Younger Dryas Impact Hypothesis (YDIH) is a theory that tries to explain the sudden cooling at the end of the Last Glacial Period, known as the Younger Dryas (YD) period. This idea was first shared in 2007 by Richard Firestone, a nuclear physicist, and his team in a scientific journal called Proceedings of the National Academy of Sciences of the United States of America. Their paper was titled "Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling." Before this, in 2001, Firestone and William Topping, an archaeologist, wrote an earlier version of the theory in a newsletter called Mammoth Trumpet.
In 2006, Firestone and others wrote a book called The Cycle of Cosmic Catastrophes, where they argued that a comet impact broke an ice dam holding back a large lake called Lake Agassiz. This caused a flood that added freshwater to the Atlantic Ocean, disrupting ocean currents and causing a sudden cooling. They also claimed the impact destabilized an ice sheet, leading to wildfires, the extinction of large animals, and the disappearance of the Clovis culture, a group of early humans. However, genetic evidence shows the Clovis people moved south and adapted to changes. Some scientists, like Vance T. Holliday, say there is no clear break in the archaeological record, and Clovis people moved frequently.
The widely accepted explanation for the freshwater entering the ocean is melting ice from retreating glaciers, as shown in a 1989 study in the journal Nature by Wallace Smith Broecker and others. James Kennett, a paleoceanographer, worked on both the 1989 study and the 2007 YDIH paper.
The 2007 paper faced criticism from scientists in many fields, including geology, astronomy, archaeology, and paleoecology. Supporters of YDIH argue that opposition comes from a small group of critics who claim a cover-up, which is unusual in science. Geologist Nicholas Pinter and others found no evidence of an impact, could not repeat key claims, and found more likely explanations for materials found. In 2011, Pinter and his team tested YDIH and concluded the hypothesis was rejected because the 2007 study's results could not be repeated and were likely misinterpretations of normal geological processes. They wrote:
In all cases, materials found were misreported as signs of an impact. No expected impact signs were found, leading to theories that were unclear, conflicting, and sometimes impossible. The YD impact hypothesis shows the importance of careful research for scientists and the public.
A key claim of YDIH was finding high levels of tiny diamonds in carbon-rich balls, which they said came from a comet hitting the atmosphere. This was the last hope for the theory until a study by physicist Tyrone Daulton found no such diamonds. Another study by geophysicist Jay Melosh showed an air burst from a comet could not create enough pressure to form the diamonds. A 2010 study by paleobotanist Andrew C. Scott found the materials were actually from fungi or insect waste. Studies in 2016 confirmed the absence of nanodiamonds.
Supporters of YDIH also cited a spike in platinum found in Greenland ice from 13,000 years ago and the 2015 discovery of the Hiawatha impact structure. However, in 2018, Melosh said the impact was unlikely to be recent, as such events happen every few million years. He also said the impact was too small to cause major extinctions. A 2025 study by volcanologist Charlotte E. Green found the platinum spike happened 45 years after the start of YD and lasted 14 years, which does not match a sudden event. Green said the platinum was from volcanic eruptions in Iceland, as shown in other studies.
Other evidence for YDIH includes black mats (layers of organic soil) and magnetic balls found at archaeological sites. A 2009 study by Jennifer Marlon, a paleoecologist, found no spike in charcoal or pollen data matching the YD boundary. Black mats appeared before the YD cooling. West, a scientist, said Marlon's dating method was incorrect, but she argued her results were consistent across multiple sites. A 2012 study by Jeffrey Pigati, a geologist, found black mats in the Atacama Desert at many sites, some older than YD. Both Marlon and Pigati said natural processes, not impacts, could explain the mats. Firestone criticized Pigati's methods, but Pigati said his findings stood. He said, "We can't disprove the impact hypothesis, but some markers are not unique to impacts." West said magnetic balls were key evidence, but a 2009 study by Todd A. Surovell found no spike in magnetic minerals at YD sites. Pinter and others later said the magnetic balls were similar to common iron-rich grains.
Public interest
The Younger Dryas Impact Hypothesis (YDIH) became widely known through its appearance in documentaries on channels such as National Geographic, the History Channel, and the PBS program NOVA. In 2024, The New York Times described YDIH as "The Comet Strike Theory That Just Won't Die" in an article that explored its history and the public's unusual interest in it. People without scientific training often believe in YDIH despite scientific evidence that contradicts it. This happens because of a mental process called "epistemic vigilance," which helps individuals tell real facts from false ones. Psychologists have found that when people receive conflicting information from experts, they often choose the side that matches their existing beliefs, political views, or cultural background. Psychologist Spencer Mermelstein explained that YDIH is appealing because it offers a simple explanation that connects Earth's geological history to a single cause and effect.
Graham Hancock, in his 2015 book Magicians of the Gods, claimed that the Younger Dryas comet caused a global catastrophe that led to the Noahide flood myth. He suggested this story was similar to flood myths from other cultures. These claims were criticized by experts such as Jason Colavito, Michael Shermer, and Marc J. Defant, who called them inaccurate. Hancock later expanded on these ideas in his 2019 book America Before: The Key to Earth's Lost Civilization, where he argued that the Younger Dryas event destroyed evidence of an advanced Ice Age civilization in North America. The Society for American Archaeology (SAA) responded with an open letter, stating that Hancock's ideas 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 its eighth episode. Physicist Mark Boslough wrote in Skeptic that many parts of the series were based on pseudoscience. Articles in The Guardian, Slate, The Nation, and other left-leaning publications criticized the series, while conservative outlets praised it and dismissed criticism as "left-wing propaganda." Political activist Tucker Carlson referred to the SAA as an "elitist, closed-minded cabal" tied to the "collapse of the American idea."
The YouTube community has also increased public interest in YDIH through many videos about the topic. Similar divisions based on identity appear in online discussions, with YouTubers who support YDIH often portraying skeptics as part of a "scientific cabal" or victims of groupthink. Despite scientific evidence against it, YDIH continues to grow in popularity because of its connection to personal and group identity, not because of scientific research. It particularly attracts people who are drawn to conspiracy theories that reject scientific expertise.