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.