The Younger Dryas impact hypothesis (YDIH) is an unpopular theory that suggests a sudden flood of freshwater into the ocean caused changes in ocean currents and led to a sudden cold period called the Younger Dryas, about 12,900 years ago. Scientists have rejected this idea for many years because there is no strong evidence to support it. Studies by experts in geology, astronomy, and other fields have shown the theory to be inconsistent, missing important information, and even contradicting basic scientific principles. Many scientists believe the debate around YDIH is unusual because its main supporters claim that most scientists disagree with it unfairly and accuse mainstream science of hiding the truth.
YDIH became more well-known through documentaries and books by author Graham Hancock, who promotes ideas that are not supported by scientific evidence. A 2022 episode of the Netflix series Ancient Apocalypse discussed Hancock’s theories 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 accepted by scientists. Supporters of YDIH often describe scientists who reject it as part of a secretive group that opposes them. People who believe in conspiracy theories are more likely to support YDIH.
Supporters of YDIH created a non-profit group called Comet Research Group Inc. to study impacts like the one proposed by YDIH. One of their most famous studies, published in 2021, claimed to find evidence of a comet impact at an archaeological site called Tell el-Hammam, which they linked to the Biblical city of Sodom. However, the study was later found to have incorrect data and was retracted in 2025 by the journal Scientific Reports. This was the second study by Comet Research Group Inc. to be retracted from the same journal.
History
The first idea that a comet hit North America at the end of the last ice age was proposed in the 19th century by Ignatius Donnelly, a politician who believed in unusual theories. He wrote a book in 1883 called Ragnarok and suggested that the comet impact caused clay and gravel deposits found across North America. He also claimed the impact destroyed a fictional lost civilization called Atlantis.
The Younger Dryas Impact Hypothesis (YDIH) is a theory that tries to explain why the Earth suddenly cooled at the end of the Last Glacial Period, known as the Younger Dryas (YD) period. This idea was first introduced in 2007 by Richard Firestone, a nuclear physicist, and his team in a scientific journal called Proceedings of the National Academy of Sciences. Earlier, in 2001, Firestone and William Topping, an archaeologist, wrote a paper titled Terrestrial Evidence of a Nuclear Catastrophe in Paleo-Indian Times for a newsletter called Mammoth Trumpet.
In their 2006 book The Cycle of Cosmic Catastrophes, Firestone and others expanded on the YDIH. However, one of the book’s authors said it was not meant to be a scientific work. They argued that the comet impact broke an ice dam holding back a large lake called Lake Agassiz, causing it to flood the Atlantic Ocean. This flood disrupted ocean currents, leading to sudden cooling. They also claimed the impact destabilized a large ice sheet called the Laurentide ice sheet, causing wildfires, the extinction of large animals, and the disappearance of the Clovis culture, a group of early people in North America. However, genetic evidence shows 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 Clovis people moved frequently, with tools like Folsom points appearing alongside 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. This idea was first proposed in a 1989 study in the journal Nature by Wallace Smith Broecker, a geochemist, and his team. 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 geology, astronomy, archaeology, and paleoecology. Supporters of YDIH claimed that opposition to the theory was from a small group of critics and that some skeptics made unfounded claims about a coverup. However, scientists like Nicholas Pinter found no evidence of a comet impact, failed to reproduce key findings, and suggested other natural explanations for scattered materials. In 2011, Pinter and others tested YDIH and concluded the hypothesis was rejected because the 2007 study’s results could not be reproduced and were likely misinterpretations of normal geological processes. They wrote that the hypothesis serves as a warning about the importance of careful scientific research.
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 suggested an air burst from a comet hitting the atmosphere over North America caused the nanodiamonds. However, a 2010 study by Andrew C. Scott, a paleobotanist, found the materials were actually fungi or insect droppings. Other studies in 2016 confirmed the absence of nanodiamonds.
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, Jay Melosh, a geophysicist, said the Hiawatha impact was unlikely to have caused a catastrophic event because it was too small and occurred too long ago. A 2025 study by Charlotte E. Green, a volcanologist, found the platinum spike happened 45 years after the start of the Younger Dryas and lasted 14 years, which does not match the sudden event YDIH suggests. Green concluded the platinum was likely 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 across North America. A 2009 study by Jennifer Marlon, a paleoecologist, examined lake sediments from 10,000 to 15,000 years ago and found no spike in charcoal or pollen that aligned with the start of the Younger Dryas. Instead, black mats appeared before the cooling event. A 2012 study by Jeffrey Pigati, a geologist, found black mats at many sites, some much older than the Younger Dryas, suggesting natural processes could explain their presence. Firestone criticized Pigati’s methods, but Pigati defended his findings, saying the materials were not uniquely caused by impacts. A 2009 study by Todd A. Surovell, an anthropologist, failed to find a spike in magnetic minerals at Younger Dryas boundary sites, unlike what Firestone’s team reported. Later studies by Nicholas Pinter and others found magnetic spherules were more likely to be iron-rich grains from natural sources rather than impact markers.
Public interest
YDIH became well-known because it was featured in documentaries on channels like National Geographic, the History Channel, and the PBS program NOVA. In 2024, The New York Times referred to YDIH as "The Comet Strike Theory That Just Won't Die" in an article that explored its history and why people find it interesting. People often believe YDIH even though scientists disagree because of a process called "epistemic vigilance," which helps individuals tell real facts from fake ones. Psychologists have found that when people receive 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 people may support YDIH because it provides a simple explanation that connects with what they know about Earth's geological history, using "one big cause, one big outcome."
Graham Hancock wrote in his 2015 book Magicians of the Gods that the Younger Dryas comet caused widespread destruction on Earth during a time cycle and that it was linked to the Noahide flood myth. He claimed this story was common in other cultures by comparing it to flood myths from different peoples. 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 an advanced Ice Age civilization in North America. The Society for American Archaeology (SAA) released an open letter stating that Hancock's ideas about this 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 Episode 8 discussed YDIH. Impact physicist Mark Boslough wrote in Skeptic that many aspects of the series are pseudoscience. Articles in The Guardian, Slate, The Nation, and other left-leaning publications criticized and mocked the series, while conservative outlets praised it and called criticism "left-wing propaganda." Political activist Tucker Carlson described the SAA as an "elitist, closed-minded cabal" connected to the "collapse of the American idea."
The YouTube community has also increased public interest in YDIH through many videos about the topic. On YouTube, similar divisions appear between people who reject or accept YDIH, similar to those in mainstream media. YouTubers who support YDIH often describe skeptics as villains part of a "scientific cabal" or victims of groupthink. Despite scientific disagreement, YDIH continues to gain attention because of its appeal to the public imagination and its connection to personal and group identity, rather than scientific research. It particularly attracts people who are drawn to conspiracy theories that reject scientific expertise.