How To Avoid The End of the World

In 1977, a geologist named Walter Alvarez was studying rock layers in Gubbio, Italy, trying to measure how long it took for ancient seafloor sediment to form. He was fixated on a thin, dark, ashy stripe called the K-T boundary—the dividing line between the age of dinosaurs and the age of mammals. Paleontologists already knew that dinosaurs had disappeared between these layers, but no one knew how quickly it happened or what caused it. Walter wasn't looking for that answer. He was just trying to date the layers. Then his father, Nobel Prize-winning physicist Louis Alvarez, called with an idea.

Louis had helped build the atomic bomb. He had flown on the observation plane behind the Enola Gay over Hiroshima, watching a city of 350,000 people vanish in a white flash. In the cockpit, he wrote his four-year-old son Walter a letter, wondering whether this terrible weapon might finally make war unthinkable. Decades later, Louis looked at a chunk of rock his son had sent him from Gubbio. The thin K-T layer struck him as strange—why was it so dark, so thin, so abrupt? He had spent his career using physics to measure trace amounts of rare elements. He knew that iridium, a metal rare on Earth's surface but common in asteroids, could serve as a time marker. When he and Walter tested the Gubbio clay, they found thirty times more iridium than expected. They tested samples from Denmark and Ne