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They were the largest creatures to inhabit the Earth, according to fossil records, and no one seems to know for sure why they suddenly disappeared. Theories abound, and one of the more popular is the Alvarez Impact Extinction Theory, formulated in 1980, which suggests that a massive asteroid hit the planet, setting off a series of catastrophic events. These include tsunamis, acid rain, severe storms, earthquakes, and volcanic eruptions. Mass Extinctions, Vol. 1: The Dinosaurs Disappear looks at possible explanations for the extinction of these animals approximately 65 million years ago.