Alan Hildebrand and William Boynton say they have found the precise location of the impact. This is a different site from the one in Cuba, which two other American geologists identified previously. Scientists have long wondered why the dinosaurs and many other species became extinct at the end of the Cretaceous period. A decade ago, however, the idea arose that the impact of a large asteroid was responsible. The evidence comes from unusually high levels of iridium in rocks 66 million years old.
Iridium is rare on Earth, though common in meteoritic material.
Geologists have also found crystals of 'shocked' quartz from the same period across North America. They claimed that these were created in the heat of an impact. Previously, researchers who thought that the impact occurred somewhere in North America, identified a crater near Manson, Iowa, as the impact point. But although the crater was of roughly the correct age, it seemed too small at only 35 kilometres across. In 1984, Hildebrand began searching for signs of a large impact in the ocean.