a gash in the earth of Russia's Far East - stretching two-thirds of a mile.
And as the planet warms up -- the Batagaika crater's permafrost is melting.
Locals call it 'the cave in' -- or 'the gateway to the underworld.'
But it has a scientific name too: a mega-slump.
Scientists warn the phenomenon is a sign of danger for what's to come.
The crater began to appear in the 1960s as a result of deforestation in the area.
The deforestation led to the loss of ground ice, which then caused the earth to begin eroding.
Since then, locals have taken note of its rapid growth:
(Erel Struchkov, Local resident and crater explorer)
"Two years ago the edge was about 20 to 30 meters away from this path. And now, apparently, it is much closer. Usually, on that side, it expands about 10 meters and on this side about four to six meters each year, it varies depending on the year."
Scientists aren't sure exactly how fast the crater will expand.
But they say Russia is warming at least 2.5 times faster than the rest of the world...
melting the long-frozen tundra that covers about 65% of the country's land, and releasing greenhouse gases stored in the soil.
Researcher Nikita Tananayev says that will further fuel global warming.
(Nikita Tananayev, Lead researcher, Melnikov Permafrost Institute in Yakutsk)
"It is a sign of danger because this is produced by higher air temperatures, by warming climate, by anthropogenic impact. So I would say that towards the future, with increasing temperatures, with higher anthropogenic pressure, we will see more and more of those mega-slumps forming, up until all the permafrost will be gone. Which is an extreme case, but I have to say it nonetheless."
Thawing permafrost has already threatened cities and towns across northern and northeastern Russia...
buckling roadways, splitting apart houses, and disrupting pipelines.
