The first major clue is the presence of iridium (a metal common in asteroids) at exactly the right place in the rocks (the Cretaceous-Tertiary or "KT" boundary, dating to 65 million years ago, which is also when non-avian dinosaurs went extinct). That was discovered around 1980 by Alvarez et al.
Further research has led us to the Chicxulub crater near the Yucatan Peninsula as the impact site.
The first major clue is the presence of iridium (a metal common in asteroids) at exactly the right place in the rocks (the Cretaceous-Tertiary or "KT" boundary, dating to 65 million years ago, which is also when non-avian dinosaurs went extinct). That was discovered around 1980 by Alvarez et al.
Further research has led us to the Chicxulub crater near the Yucatan Peninsula as the impact site.
Now you know.