Let's talk about the three major theories to explain the Cretaceous-Tertiary mass extinction. The first theory involves climate change. Global temperatures fell at the end of the Cretaceous and the seasons that we know today were starting to become more obvious. There were changes in sea levels as well as more pronounced differences in summer and winter temperatures. However, while climate change could clearly affect the survival or extinction of living species, it seems unlikely that this would bring about a sudden demise, which brings me to the second major theory: extensive volcanic activity. At the end of the Cretaceous, several parts of the world were experiencing huge amounts of volcanic activity. The best evidence of this is the Deccan Traps, a massive set of flood basalts located in eastern India.
The third, and most dramatic, theory involves the impact of a comet or asteroid. There are several lines of evidence to support this theory including the discovery of the K-T Boundary, which contains large amounts of Iridium. This element is rare on Earth but common in asteroids. In 1990, what's though to be the crater caused by this impact was found off the coast of Mexico's Yucatan Peninsula.