Located in Derweze, Turkmenistan.
While drilling in 1971 geologists accidentally found an underground cavern filled with natural gas. The ground beneath the drilling rig collapsed, leaving a large hole with a diameter of about 50-100 meters. To avoid poisonous gas discharge, it was decided to burn the gas. Geologists had hoped the fire would go out in a few days but it has been burning ever since. Locals have named the cavern The Door to Hell.