Doomsday predictions suggest that the world will end due to a religious rapture, the collision of Earth with an interstellar object, solar flares that incinerate the planet, floods, the realignment of the Earth's magnetic poles, poisonous cyanogen gas from the tail of Halley's Comet and black holes engulfing the planet. One of the more recent doomsday predictions, Y2K, caused the U.S. government to spend $108.8 billion in preventative measures.
Another popular doomsday prediction stated that the end of the world would be brought about when a rogue planet called Nibiru collided with the Earth. The date of impact would be December 21, 2012, which coincided with the end of one of the great cycles of the Maya calendar.
In 1954, Dorothy Martin said that she had been receiving messages from aliens from the planet Clarion. Martin announced that the messages bespoke of a world-ending flood that would bring about the end of civilization and that the only way to avoid the end of the world was to be saved by the aliens and their spaceships.
In 1910, members of Chicago's Yerkes Observatory warned the public that the tail of Halley's Comet was made of poisonous gas and when it passed by Earth on May 18, untold numbers of people would be killed.