Gas cave in Derweze

In 1971 geologists discovered a large underground cave filled with natural gas while drilling in the Turkmenistan village of Derweze. The ground beneath the drilling rig collapsed, leaving a large hole with a diameter of about 50-100 meters. To prevent poisonous gas from leaking out, it was decided to burn the gas, and the cave has been burning ever since.[huanqiu.com]