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Permafrost, an icy expanse of frozen ground covering one-quarter of the Northern Hemisphere, is thawing. As it does, microscopic animals are waking up and feeding on the previously frozen carbon stored in plant and animal remains, releasing heat-trapping gases as a byproduct. These gases warm the atmosphere further, melting more permafrost in a dangerous feedback loop. With permafrost containing twice as much carbon as the atmosphere, its thaw could release 150 billion tons of carbon by the end of the century.