Science
A thin layer of clay found the asteroid that killed the dinosaurs
Geologist Walter Alvarez and his physicist father Luis noticed a razor-thin clay layer worldwide, dated to exactly when the dinosaurs vanished, packed with iridium, an element rare on Earth but common in asteroids. They proposed a massive impact triggered the extinction. Confirmation came later with the discovery of the 66-million-year-old Chicxulub crater buried off Mexico's coast.