Super Volcano.

If you have read ST, the last time Toba blew its top was 75,000 years ago and it was the largest in 2 million years.
In light of recent tremors, someone just said that maybe.... maybe Toba might squirt big time again.
http://www.andaman.org/book/app-r/textr.htm

Here's a good read about Toba if you want. Quite devastating this thing is.