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1.  Volcanoes – enough debris in the atmosphere to block the sun for years
2.  Earthquakes – earth vomits
3.  Floods – a big Tsunami is common among the Mayans, the Bible and Nostradamus
4.  Asteroids – another one will hit earth someday
5.  Mankind – we take ourselves out with nukes, disease, robots
6.  Global Warming – we and the sun take mankind out
7.  Ice Age – the earth enters an ice age
8.  Earth’s Core – the spin gets jostled and earth explodes from the inside
9.  Galactic Alignments – Sun spots, magnetic pole changes, solar system wobble
10. Creators – they planned to end it all along, how did the Mayans know?
11. Aliens – note how much sci-fi has turned into reality
12. Unidentified – something from outside the universe whacks us