Alvarez finds evidence of dinosaur-killing asteroid
In 1980 physicist Luis Alvarez and his son, geologist Walter Alvarez, both of the University of California, were working together on a geology expedition in Italy. They accidentally discovered a band of sedimentary rock that contained unusually high levels of a rare element, iridium. Chemical dating techniques put the rock at around 65 million years old. Coincidentally -- or not, that is around the time the dinosaurs died out.
The Alvarezes hypothesized that the iridium, which was in a very even, widespread distribution (not just in Italy), was the result of a giant asteroid that hit Earth, sending smoke, dust, and iridium into the atmosphere. That smokescreen blocked the sun, lowering the earth's temperature, killing plants (but not seeds or roots), and eventually many species of animals, including dinosaurs. The plant-eaters died out first, followed by the meat-eaters who would have eaten them. Smaller mammals and birds could survive the cold, desolate period because of their fur, feathers, and ability to eat seeds, roots, and decaying vegetation. The pollution eventually settled to the ground, forming a thin layer of iridium.
The theory is still subject to debate.