Dr. Walter Alvarez of UC Berkeley and Dr. Premoli Silva in Italy, examining the place where the top Cretaceous and base of the Tertiary layer are mixed in an old submarine landslide. In the early 1980s Alvarez discovered a worldwide layer of clay with unusually high iridium content and postulated that it had been deposited by a large asteroid impact which caused the extinction of the dinosaurs. At the time this was a controversial theory (scientists followed Darwin's theory that extinction, like evolution, was a gradual process, unlikely to be caused by a single catastrophe) but now it is considered the most plausible explanation of the dinosaurs' abrupt demise.