"Darvaza, Turkmenistan - 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. To avoid poisonous gas discharge, it was decided to be burned 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.'"