The Door to Hell

Smithsonian features the fiery sinkhole in Turkmenistan:

So how did this fiery inferno end up in the middle of a desert in Turkmenistan? In 1971, when the republic was still part of the Soviet Union, a group of Soviet geologists went to the Karakum in search of oil fields. They found what they thought to be a substantial oil field and began drilling. Unfortunately for the scientists, they were drilling on top of a cavernous pocket of natural gas which couldn't support the weight of their equipment. The site collapsed, taking their equipment along with it—and the event triggered the crumbly sedimentary rock of the desert to collapse in other places too, creating a domino-effect that resulted in several open craters by the time all was said and done.
The largest of these craters measures about 230-feet across and 65-feet deep. Reportedly, no one was injured in the collapse, but the scientists soon had another problem on their hands: the natural gas escaping from the crater. Natural gas is composed mostly of methane, which, though not toxic, does displace oxygen, making it difficult to breathe. This wasn't so much an issue for the scientists, but for the animals that call the Karakum Desert home—shortly after the collapse, animals roaming the area began to die. The escaping methane also posed dangers due to its flammability—there needs to be just five percent methane in the air for an explosion to potentially take place. So the scientists decided to light the crater on fire, hoping that all the dangerous natural gas would burn away in a few weeks' time.
It's not as outlandish as it sounds—in oil and natural gas drilling operations, this happens all the time to natural gas that can't be captured. Unlike oil, which can be stored in tanks indefinitely after drilling, natural gas needs to be immediately processed—if there's an excess of natural gas that can't be piped to a processing facility, drillers often burn the natural gas to get rid of it. It's a process called "flaring," and it wastes almost a million dollars of worth of natural gas each day in North Dakota alone.
But unlike drillers in North Dakota or elsewhere, the scientists in Turkmenistan weren't dealing with a measured amount of natural gas—scientists still don't know just how much natural gas is feeding the burning crater—so what was supposed to be a few-week burn has turned into almost a half-century-long desert bonfire.

Previously covered here and here.