The origin of Turkmenistan's Darvaza Crater – nicknamed the "door to hell" – is disputed, but the theory most widely accepted involves a Soviet expedition to explore for gas.
A Turkmen geologist claims the borehole was set alight in 1971 after fears it was emitting poisonous gases. It has now been burning for 40 years.
The crater, which is 69 metres wide and 30 metres deep, is located in a natural gas field in Ahal Province in Turkmenistan, which has the sixth largest reserves in the world.
George Kourounis, a Canadian explorer, became the first person known to have ventured into the pit last year, though footage of his expedition first aired this week on National Geographic, which partly funded the expedition. The aim was to collect soil samples from the bottom of the pit, to try to establish whether life can exist in such a harsh environment.
"The story behind how [the crater] came into existence has been sort of shrouded in mystery, and there's no other place like it on Earth," Kourounis told National Geographic. "It is very unique, in that there's no other place where there is this pit of burning methane that's being ejected from the ground at high pressure".
Nowadays the site of the pit has become a significant tourist attraction with visitors from all over the world traveling to see it for themselves. It isn't clear how long the spectacle will last before the fire goes out.
