EDIT: Better link (provided by user georgiarepublic) with some additional photos and a video:
The Door to Hell
The Door to Hell, as locals call it, is situated near the small town of Darvaz in Uzbekistan. Thirty-five years ago, geologists were drilling for gas when then encountered a very large cavern underground filled with a poisonous gas. They ignited the gas expecting it to burn off in a few hours. Thirty-five years later, the gas is still burning.