A short drive north of Fairbanks, Alaska, there's a red shed stuck right up against a hillside. The shed looks unremarkable, except for the door. It looks like a door to a walk-in freezer, with thick insulation and a heavy latch. Whatever is behind that door needs to stay very cold.
"Are you ready to go inside?" asks Dr. Thomas Douglas, a geochemist at the U.S. Army Corps of Engineers.
Behind the door is a geological time bomb, scientists say. No one knows exactly how big the bomb is. It may even be a dud that barely detonates. But the fallout could be so large that it's felt all around the world. Now there's evidence that, in the past few years, the bomb's timer has started ticking.
Douglas opens the shed door, and we step inside. Immediately, we're standing 40 feet below ground, inside a tunnel carved into the hillside.
"That's a mammoth leg right there," Douglas says as he points to a giant femur protruding from the tunnel wall.
All around are signs of extinct creatures. Tusks poke out of the ceiling and skulls stick up from the floor. But it's the material between the bones that interests Douglas the most: the permafrost.
In the 1960s, the Army dug the tunnel so it could study this unique surface, which covers about a quarter of the Northern Hemisphere. In some places, the frozen soil extends downward more than 1,000 feet, or about the height of the Empire State Building.