An explanation of the end of the dinosaurs, by the Berkeley geologist who helped promulgate the theory. At first widely doubted, the idea is now gospel: Sixty-five million years ago, a comet or asteroid about the size of Mt. Everest slammed into the YucatÃŠn not far from present-day CancÂ£n, wiping out at least half of the species on earth. Alvarez was part of the team that discovered the first evidence of this impact, which caused an unimaginably immense explosion. Some species may have been largely eliminated immediately; others disappeared more gradually. This collision can be proven because of the immense impact crater (discovered in 1950) and dated because of the otherwise anomalous appearance of the rare element iridium (a byproduct of the explosion) at a certain stratum in core samples retrieved from the ocean floor. The iridium, in lesser amounts, can be found worldwide, notably at the Gubbio site in Italy; its presence testifies to the terrible cloud that obscured the sun after the impact, killing plants and the species that subsisted on them--as well as the meat-eating animals that preyed on the plant-eaters. While the dinosaurs could not survive the upheaval, small burrowing animals, aquatic life, and birds (thought by some scientists to be related to dinosaurs) had a better chance for survival, since they could more easily hide or flee during the dark time. Alvarez does a wonderful job of explaining these events, describing the expeditions into the rough backcountry of the YucatÃŠn in search of evidence, the growing excitement as proof of the controversial thesis emerged, and the acceptance of the theory by the scientific establishment Appealing and accessible, an excellent introduction to the subject.