Derweze underground is naturally rich in natural gas. In 1971, during a drilling, geologists accidentally found an underground cavern filled with natural gas. The ground on which the drilling rig was placed collapsed, leaving a large gaping hole exposed with a diameter of about 50-100 meters. To avoid poisonous gases coming out of the hole, it was decided to let the gases burn. As of 2008, gases in the underground cavern are still burning without interruption. Locals have named the cavern The Door to Hell.
Okay, obviously I was wrong about hell. It does exist, and all you non believers can go to h.. Turkmenistan. 😉