Iridium is a very rare element in the Earth's crust, but is found in anomalously high concentrations (around 1000 times greater than normal) in a thin worldwide layer of clay marking the boundary between the Cretaceous and Paleogene periods 66 million years ago. This boundary marks a major extinction event, including extinction of the dinosaurs along with about 70% of all other species. In 1979 the physicist Luis Alvarez, his son, geologist Walter Alvarez, and chemists Frank Asaro and Helen Vaughn Michel were the first to link the extinction to an extraterrestrial impact based on the observation that iridium is much more abundant in meteorites than it is on Earth. During 1979 dozens of newspapers and magazine articles presented the original Alvarez hypothesis on the basis of only a talk at the American Geophysical Union meeting (Washington, May 1979) and accompanying abstract cited here.

(Thanks to Juan Weiss for this reference and its interpretation.)