5. The Door to Hell, Derweze
The Door to Hell is a natural gas field in Derweze,Turkmenistan, that collapsed into an underground cavern in 1971, becoming a natural gas crater. Geologists set it on fire to prevent the spread of deadly methane gas, and it has been burning continuously since then. The diameter of the crater is 69 m, and its depth is 30 m.
The site was identified by Soviet engineers in 1971. It was originally thought to be a substantial oil field site. The engineers set up a drilling rig and camp nearby, and started drilling operations to assess the quantity of oil available at the site. When they instead found gas, the ground beneath the drilling rig and camp collapsed into a wide crater and disappeared.
Expecting dangerous releases of poisonous gases from the cavern into nearby towns, the engineers thought it best to burn the gas off. It was estimated that the gas would burn out within a few weeks, but it has instead continued to burn for more than four decades.