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He also points to independent research by William Napier at the University of Cardiff in the United Kingdom that indicates such explosions could have come from a debris trail created by Comet Encke, which also would not have left a crater.A large rock plunging into the Earth's atmosphere may detonate in the air without coming into contact with the ground.Such an explosion occurred in Siberia in the early 20th century; the explosive energy of the so-called Tunguska event was more than 1,000 times more powerful than the atomic bomb dropped on Hiroshima.

"If a four-kilometer [2.5-mile] comet had broken up over North America only 12.9 thousand years ago, it is certain that it would have left an unambiguous impact crater or craters, as well as unambiguous shocked materials," Boslough said.

Boslough, who has spent decades studying the effects of comet and asteroid collisions, was part of a team that predicted the visibility of plumes from the impact of the 1994 Shoemaker-Levy 9 comet with Jupiter.

"Comet impacts may be low enough in density not to leave craters," Firestone told Live Science by email.

"Despite more than four years of trying by many qualified researchers, no unambiguous evidence has been found [of such an event]," Mark Boslough, a physicist at Sandia National Laboratories in New Mexico, told Live Science.

"That lack of evidence is therefore evidence of absence." Changing times Almost 13,000 years ago, a prehistoric Paleo-Indian group known as the Clovis culture suffered its demise at the same time the region underwent significant climate cooling known as the Younger Dryas.

Animals such as ground sloths, camels and mammoths were wiped out in North America around the same period.[Wipe Out: The 10 Most Mysterious Extinctions] In 2007, a team of scientists led by Richard Firestone of the Lawrence Berkeley National Laboratory in California suggested these changes were the result of a collision or explosion of an enormous comet or asteroid, pointing to a carbon-rich black layer at a number of sites across North America.A 130-foot-meteor created the mile-wide Meteor Crater in Arizona.The comet proposed to have impacted life in North America was significantly larger, but no crater indicating its collision has been found.A comet crashing into the Earth some 13,000 years ago was thought to have spelled doom to a group of early North American people, and possibly the extinction of ice age beasts in the region.But the space rock was wrongly accused, according to a group of 16 scientists in fields ranging from archaeology to crystallography to physics, who have offered counterevidence to the existence of such a collision.