Layers of basalt lava, called the Deccan traps. These voluminous lava flows cover over 500,000 square kilometers of India. They erupted 65 million years ago. In 1980, Walter Alvarez and his colleagues at the University of California, Berkeley, advanced the idea that an asteroid whacked into the Earth causing the end of the Cretaceous Period. They had found exceptionally high concentrations of the element iridium at the boundary between the Cretaceous and Tertiary periods.
Iridium is rare in the crust of the Earth, but much higher in abundance in meteorites.
This led Alvarez to propose that the enrichment is due to an asteroid or comet impact. Since then geochemists have found iridium anomalies at the Cretaceous-Tertiary boundary worldwide. It is often accompanied by shock-damaged quartz, an unequivocal indicator of meteorite impact. To top it off, in 1991 A. R. Hildebrand and his colleagues identified the crater buried under a kilometer of sediments. Called Chicxulub, it straddles the Yucatan peninsula and the Gulf of Mexico.