As its name suggests, permafrost is meant to remain permanently frozen.
But Arctic soil is rapidly melting — twice as fast as other parts of the world.
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Carbon dioxide, which contributes to global warming, is locked inside permafrost.
As it melts, the gas will be released. NASA says permafrost could become a permanent source of carbon by 2100.
Arctic craters are opening up due to another gas — methane — building up underground.
Siberia's Yamal peninsula saw a 160-foot-deep hole erupt in August.
Wetter, softer ground means buildings and other structures will crumble.
In May, a Russian oil tank spilled, polluting Ambarnaya and Daldykan rivers and turning them red.
Ancient landscapes and animals — sometimes perfectly preserved — are revealed when the icy tundra melts.
Tap to see Dogor, an 18,000-year-old puppy found in Siberia. >>
Maybe the freakiest element exposed by melting land? Diseases.
Anthrax escaped permafrost in 2016. It made 13 people sick and killed more than 2,000 reindeer.
And in 2014 researchers reported a giant, still-infectious, 30,000-year-old virus in Siberian ice. (It can only infect amoebas, not humans.)
Read more stories about what's lurking beneath the ice.