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Chicxulub Crater, Mexico.
Image source: NASA's Astronomy Picture of the Day, 26 Feb. 2000

One of the most well known debates in science is that over what caused the extinction of the dinosaurs. During the extinction about 65 million years ago, dinosaurs weren't the only creatures to perish: scientists estimate that over 70% of all life on earth came to an end. This widespread loss of diversity is referred to as the K-T event, or Cretaceous-Tertiary Mass Extinction, so named because of its place on the geologic time scale at the very end of the Cretaceous period.

Explanations for the extinction were for many years provided by two competing theories: that the global effects of either increased volcanism or the effects of a large object (asteroid or comet) impact created conditions too harsh for most life on earth. Chicxulub crater, first detected by offshore oil exploration conducted by Petroleos Mexicanos in the 1970s was not immediately identified as a crater, due in part to its burial beneath a kilometer of Cenozoic limestones. It wasn't until the early 1990s that it was recognized as the site of a massive impact, at least 180 km (112 miles) wide, 65 million years ago. Estimates put the size of the asteroid or comet at 10 to 20 km (6.2 to 12.4 miles) in width. Its impact would have created huge tsunamis, earthquakes, and winds; it would also have released gases and debris into the atmosphere, thereby darkening the sky, lowering temperatures and creating acid rain. The temporal correlation combined with strong geochemical evidence, lends great weight to the impact theory as the cause of the K-T event.