In the late 1970s, several research groups were trying to determine the cause of the boundary between the geological layers corresponding to the Cretaceous and Tertiary periods. About 65 million years ago. At this boundary, called the K/T boundary (where "K" represents the German spelling of "Cretaceous" and "T" represents "Tertiary"), a sudden change in the fossils that are found in the rocks means that a substantial fraction of all living species on Earth died out. Included in this extinction event were the dinosaurs (the most famous of the extinctions), most land-dwelling animals, and a significant fraction of the marine invertebrate species. In fact, this sudden extinction event defines the boundary between the two geologic periods. In most places where the K/T boundary is seen in the geologic record (that is, where erosion has not destroyed it), it is marked by the presence of a layer of clay that can be up to several meters thick.

The scientists measured the abundance of what are called siderophile elements within the clay. Siderophile elements are of interest because they are not very abundant within Earth's crust-they tended to remain with the iron and nickel when iron and nickel concentrated into the planet's core early in Earth's history. However, they are abundant in meteorites, which. unlike Earth, never differentiated into a core, mantle, and crust. One of the siderophiles is the element iridium. Within the clay layer, the amount of iridium was found to be ten times that of adjacent layers of rock. This was such an enormous increase that the scientists postulated that a large influx of extraterrestrial material must have occurred at that time. In addition to the iridium and other siderophile elements, the scientists noted that the boundary-layer clay contained grains of the mineral quartz that had been shocked by high pressure and temperature into a denser form. The dense forms of quartz. called coesite and stishovite, occur almost exclusively as a result of the tremendous forces that result from the impact of an asteroid onto Earth's surface: they are found in abundance at Meteor Crater in Arizona in the United States, for example.

The iridium excess and the presence of shocked quartz suggested that an impact of a single, large asteroid onto the surface of Earth had occurred at the time of the K/T boundary. Based on the worldwide distribution of the clay layer, and on the abundance of iridium within it, it was suggested that the impact of a single, 10kilometer-sized object could have been responsible. A 10-kilometer object impacting onto the surface of Earth would produce a crater that was almost 100 kilometers across. Unfortunately, there was no known 100-kilometer crater that had formed 65 million years ago. The only known crater of that age was the Manson impact crater in lowa in the United States; this crater is only 10 kilometers across, however, and is much too small to account for the observed amount of iridium.

There were no guarantees that the search for a crater would be successful. The crater, if it existed, might have been obliterated by erosion; and there was about a 40 percent chance that it had formed on the ocean floor and subsequently had been submerged beneath the continents during the 65 million years since its formation. However, in a remarkable confirmation of the hypothesis, the crater was found, located just off the Yucatán peninsula in Mexico. Known as Chicxulub, it is about 200 kilometers in diameter and lies half on land and half on the ocean floor. Lying half on the ocean floor, and having been weathered substantially where it lies above the surface, it had not been identified prior to the search for the K/T impact crater There is a suggestion in the gravity and topography data that the crater might actually have been about 3,000 kilometers across. This large crater would have been formed by the impact of an asteroid 10 to 20 kilometers across.