Abstract: The extinction of the dinosaurs sixty five million years ago has been one of the most enduring geologic mysteries. Theories abound, but much recent evidence supports the idea that the dinosaurs, along with 75% of life on the earth, disappeared in the catastrophic wake of the collision of a 10-km diameter giant asteroid or comet with the planet. The evidence ranges from recovery of tiny micro-diamonds in deep-sea deposits to scientific sleuthing that resulted in the discovery of a giant impact crater on Mexico's Yucatán Peninsula.

But the story is even more far-reaching. The dinosaur mass extinction is just one of a score of major extinction events in earth history, and evidence is converging on the conclusion that these catastrophes too were the result of large comet impacts. The impacts may have been related to periodic showers of comets paced by the motions of the Solar System through the Galaxy. If true, a new and revolutionary view of our planet and the history of life may be emerging.