The collision of a giant meteorite with the earth 66 million years ago that caused catastrophic changes to the earth’s climate and environment and a mass extinction of species, including the dinosaurs. This hypothesis was advanced in 1980 by the US physicist Luis Walter Alvarez (1911–88) and his geologist son Walter Jr, based on the unusually high concentration of the element iridium in a thin layer of clay deposited at the end of the Cretaceous (see iridium anomaly). This clay marks the boundary between the Cretaceous period and the more recent Palaeogene (the so-called K-T boundary). Subsequently, geologists discovered a possible impact crater, roughly 160 km in diameter, along the coast of eastern Mexico, and other evidence has tended to support the hypothesis. Such a collision would have produced a massive tidal wave and fireball and sent a vast cloud of rock and other debris into the atmosphere. The resulting upheaval in the climate is estimated to have caused the extinction of some 75% of all species.

http://serc.carleton.edu/research_education/cretaceous/ktboundary.html Resources detailing the evidence and main theories regarding mass extinctions and the Alvarez event