
It's a large natural gas deposit in Darvasa, Turkmenitan. In 1971, geologists who were drilling in the area caused a large collapse, and found the deposit. At the time, they felt that burning the poisonous gas would eradicate the threat in a few hours.
Cue 37 years later:
http://www.youtube.com/watch?v=KGHxVW1NbrQ
http://en.wikipedia.org/wiki/Door_to_hell
http://johnhbradley.com/pictures2.asp?var=070707darvaza
~HotShotX