Stunning drone footage has revealed details of the Batagaika crater, a one-kilometre-long gash in Russia's Far East that forms the world's biggest permafrost crater.
In the video two explorers clamber across uneven terrain at the base of the depression, marked by irregular surfaces and small hummocks, which began to form after the surrounding forest was cleared in the 1960s and the permafrost underground began to melt, causing the land to sink.
"We locals call it 'the cave-in,'" local resident and crater explorer Erel Struchkov told Reuters as he stood on the crater's rim. "It developed in the 1970s, first as a ravine. Then by thawing in the heat of sunny days, it started to expand."
