Researchers predicted that somewhere there had to be a crater about 150 miles across. At the same time, Petroleo Mexico was drilling in the northern Yucatan Peninsula, through about half a kilometer of limestone sediment. At the bottom of the core, the drill hit something hard: a dense, green, glass-like material.
The first assumption was that it was volcanic. Then a giant circular structure became visible under the limestone. The green glass turned out to be rock melted by the superheated temperatures of the impact.