The Derweze area is rich in natural gas. While drilling in 1971, geologists tapped into a cavern filled with natural gas. The ground beneath the drilling rig collapsed, leaving a large hole with a diameter of about 70 metres (230 ft) at 40°15′10″N 58°26′22″E. To avoid poisonous gas discharge, it was decided to burn it off. Geologists had hoped the fire would use all the fuel in a matter of days, but the gas still burns 40 years later. Locals have dubbed the cavern "The Door to Hell".

Via [Solid Planet]