In the mid-1970's, Walter Alvarez, a geologist, was studying Earth's polarity. It had recently been learned that the orientation of the planet's magnetic field reverses, so that every so often, in effect, south becomes north and vice versa. Alvarez and some colleagues had found that a certain formation of pinkish limestone in Italy, known as the scaglia rossa, recorded these occasional reversals. The limestone also contained the fossilized remains of millions of tiny sea creatures called foraminifera. Alvarez became interested in a thin layer of clay in the limestone that seemed to have been laid down around the end of the Cretaceous Period. Below the layer, certain species of foraminifera-or forams, for short-were preserved. In the clay layer, there were no forams. Above the layer, the earlier species disappeared and new forams appeared. Having been taught the uniformitarian view, which held that any apparent extinctions throughout geological time resulted from `the incompleteness of the fossil record` rather than an actual extinction, Alvarez was not sure what to make of the lacuna in geological time corresponding to the missing foraminifera, because the change looked very abrupt.

Had Walter Alvarez not asked his father, the Nobel Prize-winning physicist Luis Alvarez, how long the clay had taken to deposit, the younger Alvarez may not have thought to use iridium, an element rarely found on earth but more plentiful in meteorites, to answer this question. Iridium, in the form of microscopic grains of cosmic dust, is constantly raining down on the planet. The Alvarezes reasoned that if the clay layer had taken a significant amount of time to deposit, it would contain detectable levels of iridium. The results were startling: far too much iridium had shown up. The Alvarez hypothesis, as it became known, was that everything-not just the clay layer-could be explained by a single event: a six-mile-wide asteroid had slammed into Earth, killing off not only the forams but also the dinosaurs and all the other organisms that went extinct at the end of the Cretaceous period.