Just what exactly is permafrost? And what is happening now that it's warming up? To find out, we enter the Arctic Circle's secret world of ice and frozen history.

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...

Just what exactly is permafrost? And what is happening now that it's warming up? To find out, we enter the Arctic Circle's secret world of ice and frozen history.

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.