Twelve‐Year Trail of Clues Leads to Impact Crater From the K–T Boundary

The search started with a hypothesis that was literally out of this world: Could a massive comet or asteroid, perhaps 10 km in diameter, have struck the Earth about 65 million years ago, changing the climate so drastically that the dinosaurs and other creatures could no longer survive? That question was raised in 1980 by Luis Alvarez, his son Walter, Frank Asaro and Helen Michel of the University of California, Berkeley. Their evidence at the time was scanty: only an anomalous concentration of iridium at the geological stratum corresponding to the era when the giant reptiles became extinct and supposedly delivered by the extraterrestrial projectile. (See the article by Luis Alvarez in PHYSICS TODAY, July 1987, page 24.) But the suggestion triggered a massive hunt, and geological sleuths soon gathered much substantiating evidence. The telltale crater, however, remained elusive.