It is understood that while drilling in 1971, the Soviet 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 69 meters and a depth of about 30 metres. To avoid poisonous gas discharge, it was decided to burn the gas. At that time it was thought that the gas will be burned away in few weeks, however it is still burning with no sign of abatement. It has now become a tourist attraction.
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Source of information: Wikipedia