I'm on and off on choices.
Why?
Yellowstone.
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You may know Yellowstone to be a very beautiful place with extraordinary scenery and breathtaking water plumes, but underneath that is a super volcano. Super volcanoes are 3 to 4 times more powerful than a normal volcano, and many are hidden in places you would not think of, such as the ocean floor. This particular volcano goes off about every 645,000 years. Well, it has a high possibility that Yellowstone could blow up, (Hence Yellowstone blowing up in the movie 2012.) which would destroy a good chunk, if not all of the United States. The ash from this explosion would for sure wipe out the rest of the country, (Washington D.C. scene in 2012) being up to several feet high. The ash would eventually spread and cover the world, and... well you get the idea. The reaction from a super volcano going off could also trigger others to go off as well.
...I feel like watching 2012 now.
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There is also solar flares to boot. (If we still make it past both of these, the sun is going to blow up in about 5 million years. It's true. It's what happens to all stars.)