The Door to Hell, is situated near the small town of Darvaz in Turkmenistan. 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. The gas is still burning to this day. Its 60 meters in diameter and 20 meters depth have not been caused by volcanic activity or a meteorite impact.This crater was created sometime in the 50’s when the Soviets were prospecting for natural gas in this area and it’s been burning since then.
Photographer John H. Bradley has more amazing photos of the Darvaz Burning Gas Crater: Link