STORY: This is the world's biggest permafrost crater...

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...