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.
Technically, permafrost is frozen soil. But it's helpful to think of it in terms of chocolate cake. Typically, cake is soft, moist and spongy. Now if you take that cake, dip it into water and freeze it, the cake becomes hard or stiff. That's exactly what happens to soil when you freeze it: Moist, soft soil turns hard and stiff. That's permafrost.
For the first time in centuries, the Arctic permafrost is beginning to change — rapidly. It's warming up. Some places are softening like a stick of butter left out on the kitchen counter.
In northern Alaska, the temperature at some permafrost sites has risen by more than 4 degrees Fahrenheit since the 1980s, the National Oceanic and Atmospheric Administration reported in November. And in recent years, many spots have reached record temperatures.
"Arctic shows no sign of returning to reliably frozen region of recent past decades," NOAA wrote in its annual Arctic Report Card last year.
The consequences of this warming could have ripple effects around the world. To explain why, Douglas takes me deeper down into the tunnel.
"This is really an amazing feature," he says, shining his flashlight up to the ceiling. Crispy grass is dangling upside-down above our heads.
"It's green grass — from 25,000 years ago," he exclaims. "It has been preserved that way for 25,000 years."
The permafrost is packed with the remains of ancient life. From prehistoric grass and trees to woolly mammoths and woolly rhinoceroses, just about every creature that lived on the tundra over the past 100,000 years is buried and preserved down in the permafrost.
And all this life is made of carbon. So there's a massive amount of carbon buried down here. "The permafrost contains twice as much carbon as is currently in Earth's atmosphere," Douglas says. "That's 1,600 billion metric tons."
In fact, there's more carbon in the permafrost, Douglas says, than all the carbon humans have spewed into the atmosphere since the Industrial Revolution — first with steam trains, then with coal plants, cars and planes.
Right now the permafrost carbon is inert and trapped in the frozen soil. But what happens when the soil thaws? That's the question Douglas and his colleagues are trying to figure out.
A few years ago, they ran a simple experiment. They brought big drills into the tunnel and cut out chunks of ice. "We collected pieces about the size of Coca-Cola cans," he says, as he points out holes in the tunnel's wall.
They took the ice back to the lab and let it slowly come up to room temperature. Then they looked for signs of life. A few days later, something started growing — slowly at first, but then like gangbusters.
"This is material that stayed frozen for 25,000 years," Douglas says. "And given the right environmental conditions, it came back alive again vigorously."
They were ancient bacteria. And once they warmed up, they were hungry. The bacteria started converting the carbon that's in dead plants and animals into gases that cause climate change: carbon dioxide and methane.
That experiment was in the lab. But imagine these bacteria waking up, all around the Arctic, across Canada, Greenland and Russia. Last year, scientists started seeing signs of this happening in northern Alaska.
"We have evidence that Alaska has changed from being a net absorber of carbon dioxide out of the atmosphere to a net exporter of the gas back to the atmosphere," says Charles Miller, a chemist at NASA's Jet Propulsion Laboratory who measures gas emissions from Arctic permafrost.
Scientists don't know yet how much carbon will get released from thawing permafrost or how fast it will happen. Some of the carbon — maybe a big percentage of it — will get washed into the ocean by erosion. Some of the carbon will also get sucked back into the ground by new trees and plants popping up across the warming tundra.
But once carbon begins to percolate up through the thawing soil, it could form a feedback loop "over which we would have zero control," Miller says. The gas, coming from the ground, warms the Earth, which in turn causes more gas to be released and more warming to occur.
Thawing permafrost is a big wild card of climate change.
DAVID GREENE, HOST:
Come with us on a journey inside the Earth. Well, inside a special layer in the Earth. A quarter of the Northern Hemisphere is covered with what's called permafrost, and for the first time in centuries, the permafrost is beginning to warm up because of climate change.
TOM DOUGLAS: And that's a mammoth bone right there.
MICHAELEEN DOUCLEFF, BYLINE: Whoa. It's just sticking out of the wall.
DOUGLAS: Just sticking out of the wall.
GREENE: As it thaws, the permafrost is unleashing something that could affect the whole world. NPR's Michaeleen Doucleff reports.
DOUCLEFF: We start off 40 feet underground inside a tunnel about as wide as an SUV. All around us are signs of extinct creatures. Tusks are sticking out from the ceiling, and a skull pokes out from the ground. Ancient bones.
DOUGLAS: Where we are here has been dated at about 14,000 years ago.
DOUCLEFF: I think this is one of the coolest places I've been to.
DOUGLAS: Really? All right. Cool. Neat.
DOUCLEFF: That's Tom Douglas, a geochemist with the U.S. Army. He says, back in the 1960s, the Army dug this tunnel so they could study the permafrost. All the walls are covered in a soft brown dust, but what's underneath is hard as concrete.
DOUGLAS: If I tap on this now, you'll see it's hard as a rock.
DOUGLAS: That's permafrost. Anything in there is frozen. You can even see the little marks on there. It's pretty hard.
DOUCLEFF: Permafrost is technically frozen soil, but think of it in terms of chocolate cake. Typically that cake is spongy, soft. But if you take that cake and dip it into water and freeze it, it turns hard. That's exactly what happens to soil when you freeze it. You get permafrost. We walk deeper into the tunnel.
DOUGLAS: All right. So keep going.
DOUCLEFF: Those woolly mammoth bones aren't the only bizarre thing hidden in permafrost. Just then we walk right through another one.
DOUGLAS: So here's a nice wedge.
DOUCLEFF: It looks like a giant wall of ice, but it's really an upside down iceberg buried in the earth, and the tunnel cuts right through it.
It's a huge chunk of ice all around us.
DOUGLAS: Yeah. I mean, it's basically the size of a house or something.
These icebergs are buried throughout Alaska. They're buried under homes, under office buildings, bridges. And they've been frozen solid for centuries, even longer. They actually hold the ground together. Here's the problem.
DOUGLAS: That's about 99 percent water ice by volume.
DOUCLEFF: So when the ground warms up...
DOUGLAS: Imagine turning that into water. You'll leave a trench in the ground that people could fall into, right?
DOUCLEFF: Or, this whole tunnel could collapse?
DOUCLEFF: That's exactly what's happening across Alaska. A study in 2016 found that these giant buried icebergs are melting rapidly. New lakes are forming in some places. They're draining in others. Rivers are appearing where they never were before, and the land is sinking. Clearly this is going to be a big problem for Alaskans and other people up North. But that's not what worries Tom Douglas the most. There's something else hidden here that could affect the whole world.
DOUGLAS: Keep going down. Watch your head.
DOUCLEFF: Douglas takes me deeper down into the tunnel.
DOUGLAS: This is really an amazing feature of the tunnel. It's the only place we see it.
DOUCLEFF: He shines his flashlight up to the ceiling.
DOUGLAS: What does that look like to you?
DOUCLEFF: Like grass.
DOUGLAS: Green grass, right?
DOUCLEFF: Yeah. It's green?
DOUGLAS: It's green grass.
DOUGLAS: Yeah. See that?
DOUCLEFF: Wait. We have to tell people that, like, the grass is actually growing down.
DOUGLAS: Upside down.
DOUGLAS: This was in ice, and had been preserved that way for 25,000 years. I mean, if I...
DOUCLEFF: Wait. This is 25,000-year-old grass?
DOUCLEFF: That's incredible.
DOUGLAS: Yeah. Really amazing.
DOUCLEFF: You see, the thing is, basically anything that's died in the Arctic over the past hundred-thousand years is buried and preserved down here. The permafrost is packed with plants, like this grass, and dead animals, like those woolly mammoths we saw earlier. All this life is made of carbon. In fact, there's a massive amount of carbon down here. There's more carbon trapped in this permafrost than all the carbon humans have spewed into the atmosphere, first with steam trains then with their cars, planes, coal plants, everything we've done since the Industrial Revolution.
DOUGLAS: The permafrost contains twice as much carbon as is currently in Earth's atmosphere, 1,600 billion metric tons.
DOUCLEFF: Right now this carbon is trapped, frozen. So the big question is what happens to this carbon as the permafrost thaws? Because, you see, there's not just dead creatures in the permafrost. Down here, we are also surrounded by something that's coming back to life.
DOUGLAS: See, that white flag right there is where you've got the 27,000-year-old material.
DOUCLEFF: A few years ago, Douglas and his colleagues ran a very simple experiment. They brought big drills into the tunnel and cut out chunks of ice.
DOUGLAS: We collected basically pieces about the size of a Coca-Cola can.
DOUCLEFF: They took the ice back to the lab...
DOUGLAS: Let it slowly come up to room temperature.
DOUCLEFF: ...And then looked for signs of life. A few days later, something started growing like gangbusters - ancient bacteria.
DOUGLAS: This is material that stayed frozen for 25,000 years old, and, given the right environmental conditions, came back alive again vigorously.
DOUCLEFF: Once the bacteria warmed up, they were hungry, and they started eating the dead plants and animals, turning their carbon into gases.
DOUGLAS: Both carbon dioxide and methane.
DOUCLEFF: Those are the two main gases that cause climate change. Now, that was in the lab. But imagine these bacteria waking up as the permafrost thaws all around the Arctic - in Canada, Greenland, Russia, here in Alaska. Charles Miller is a chemist at NASA's Jet Propulsion Laboratory who studies permafrost. He says that in the past few years they've started seeing the microbes here waking up, warming up and releasing gases.
CHARLES MILLER: There's been quite a tremendous change in the temperature of the permafrost. This warming is causing carbon dioxide to be liberated from the land surface so we see a net release of carbon from the land back to the atmosphere.
DOUCLEFF: Miller says they don't know yet how much carbon will get released from thawing permafrost or how fast it will happen. It's a big wild card of climate change. But once gases start coming off, it could form this type of feedback loop.
MILLER: Over which we would have essentially zero control.
DOUCLEFF: Where the gas coming from the ground warms the Earth, in turn causing more gas to be released, and more and more warming.
Michaeleen Doucleff, NPR News. Transcript provided by NPR, Copyright NPR.