While mass extinctions of numerous animal species have occurred throughout geologic history, it is the unexplainable disappearance of the great dinosaurs at the end of the Cretaceous Period (approximately 66 million years ago) that has generated a tremendous amount of debate. Some scientists speculate that severe changes in the Earth's environment were the cause, while others surmise receding seas and widespread disease. Still others contend that the dinosaurs' demise might have been the result of a widespread disaster. However, consensus has not yet been achieved.

In 1980, a disaster began to look more likely. A team of scientists headed by Luis Alvarez (1911-1988), who received the 1968 Nobel Prize in Physics, proposed the Cretaceous impact theory, which asserts that an enormous asteroid impact marked the end of the Cretaceous Period, causing the extinction of the dinosaurs. Alvarez, his son, Walter Alvarez (1940-) and scientists Frank Asaro and Helen Michel...