After centuries of investigation, the reason dinosaurs vanished remains one of the most perplexing questions in all of science. After all, they dominated the world for over 140 million years. How these strong, adaptable creatures became extinct is a subject that has filled books, magazines, movies and dreams. Many theories have been developed. Beginning in 1980, Nobel-prize-winning physicist Luis Alvarez suggested that a massive comet or meteor was to blame. Other scientists pointed to scenarios such as violent volcanic activity, or perhaps a virulent dinosaur disease. Discover the latest theories about what killed the "Terrible Lizards," an event that left the earth ripe for the rise of another amazing species...human beings.