…the dinosaurs got wiped out. So naturally we would be interested in seeing if there's anything strange about that layer. What Alvarez found is that there was an anomalously high abundance of certain atoms, of which the chief one is iridium, which is…

…iridium remains distributed. So meteorites that come from asteroids have lots more iridium than the surface of the earth. So the Alvarez Contention was that the reason there's a lot of iridium in here is because an extraterrestrial object hit the earth and…

…of places. You dig down in the sedimentary column there and you find excess iridium. The impacting object had to be Alvarez calculated about 10 kilometers across that is the size of Halley's Comet and therefore the Berkeley Group proposed that 65 million…