A doomsday event is one or more catastrophic actions occurring or affecting the earth and the life it harbors. It includes scientific reasons such as an asteroid collision or nuclear warfare, and non-scientific reasons such as an alien invasion or a religious apocalypse.
A common theory on the extinction of the dinosaurs postulates a collision with a large asteroid which would result in a significant lowering of worldwide temperatures as dust blocks solar energy. Evidence for this theory include a layer of iridium at the K-T boundary layer, and a large crater in the area of Chicxulub, Mexico.
Many religions include beliefs pertaining to the end of time. The Germanic tribes believed that the world would end in a tremendous battle of the gods known as Ragnarok. In the Christian Bible, the Book of Revelation describes Armageddon, a final battle between the forces of God and the forces of Satan.