You can find evidence of a changing climate everywhere on Earth. But nowhere are the changes more dramatic than in the Arctic.

Our world’s northern polar region is warming twice as fast as the global average. And the consequences are easy to spot. On average, Arctic sea ice extent is shrinking every summer. The Greenland ice sheet is becoming unstable.

But perhaps most disturbing are the changes occurring underground in the permafrost. Permafrost is a layer of frozen soil that covers 25 percent of the Northern Hemisphere. It acts like a giant freezer, keeping microbes, carbon, and soil locked in place.

Now it’s melting. And things are getting weird and creepy: The ground warps, folds, and caves. Roadways built on top of permafrost have becoming wavy roller coasters through the tundra. And long-dormant microbes — some trapped in the ice for tens of thousands of years — are beginning to wake up, releasing equally ancient C02, and could potentially come to infect humans with deadly diseases.

A recent Arctic Council report says that 20 percent of the permafrost near the surface may melt by 2040. Already scientists have noticed permafrost temperatures slowly climbing. “In the 1980s, the temperature of permafrost in Alaska, Russia and other Arctic regions averaged to be almost 18°F,” the U.S. Geological Survey explained in 2015. “Now the average is just over 28°F.”

A 2010 study in Russia found 0.5°C to 2°C of permafrost warming in the past three decades, bringing some locations dangerously close to thawing and pushing the boundary of permafrost regions ever northward. And the depth of the “active layer” — the top layer of permafrost that thaws in the summertime — is growing deeper in the Arctic regions north of Europe, a sign of instability.

When it thaws, a Pandora’s box is unleashed. To better understand the strange changes in the permafrost, I spoke with Robert Max Holmes, an earth systems scientist with the Woods Hole Research Center. When I reached him by phone, he was in Bethel, Alaska, a small outpost town 400 miles west of Anchorage, and had just come back from an eight-day research and teaching expedition in the wilderness.

A week earlier, Holmes and his students had set up temperature sensors in the soil near their encampment. Their first reading was 0.3°C. “It’s barely frozen. And we just sort of sat there stunned. You don’t know whether to cry or what. Because you’re just like: My God, this whole thing is just going to change in a big way.”

Here’s how.

1) Permafrost has been frozen for millennia. Thawing it is a huge disruption.

The simplest definition of permafrost is ground that has been frozen for at least two years.

But it’s so much more than that. In much of the Arctic, that ground has been frozen for tens of thousands of years. And a huge amount of it is frozen — permafrost rests in 25 percent of all the land area in the Northern Hemisphere.

The top few inches (up to a few feet) of the permafrost is what’s known as the “active layer.” This topsoil does thaw with yearly seasonal changes, and is home to a thriving ecosystem. So how do scientists know there’s permafrost underneath it?

“We have these things called thaw depth probes, which is basically just a T-bar, a steel rod that’s a centimeter in diameter and 1.5 meters or so long,” Holmes says. They poke the ground with it. “It’s like pushing a knife through warm butter or something, and then you hit the bottom of the tray, and boom” — there’s your permafrost.

Eventually, if you dig deep enough, the permafrost again thaws due to heat from the Earth’s core.

Permafrost is like the bedrock of the Arctic (you literally need jackhammers to break it apart). But rising air temperatures in the region are chipping away at this bedrock.

“Half the volume of permafrost may be frozen water,” Holmes says. “When that thaws, the water just runs off. The water may head downhill or the water has a lower volume than is ice, so the ground just slumps and kind of falls apart.”

(The New York Times has a great new interactive showing how much permafrost in the Alaska may inevitably melt.)

2) The biggest threat is carbon

You can think of the Arctic permafrost as a giant kitchen freezer.

If you put organic (carbon-based) matter in your freezer, the food will stay intact. But if the freezer compressor breaks, it will slowly heat up. As it heats up, bacteria begin to eat your food. The bacteria make the food go rotten. And as the bacteria consume the food, they produce carbon dioxide, methane, and other gases and chemicals that smell terrible.

For tens of thousands of years, permafrost has acted like a freezer, keeping 1,400 gigatons (billion tons) of plant matter carbon trapped in the soil. (That’s more than double the amount of carbon currently in the atmosphere.) Some of the plant matter is more recent, and some is from glacial ice ages that radically transformed a lush landscape into a tundra.

“Plants are growing in permafrost regions, and when those plants die, because of the cold temperature, they don’t fully decompose, so some of that organic carbon is left behind,” Holmes says. When the permafrost thaws, “it starts to rot, it starts to decompose, and that’s what’s releasing carbon dioxide and methane,” he says.

This is one reason scientists are so worried about a melting Arctic: When the bacteria turn the carbon in the Arctic into C02 and methane, it accelerates a feedback loop. The more methane and carbon released, the more warming. The more warming … you get it.

A 2014 study in Environmental Research Letters estimates that thawing permafrost could release around 120 gigatons of carbon into the atmosphere by 2100, resulting in 0.29°C of additional warming (give or take 0.21°C). By 2300, another study in Nature Geoscience concludes, the melting permafrost and its resulting carbon feedback loops could contribute to 1.69°C of warming. (That’s on the high end. It could be as low as 0.13°C of warming.)

But these are just estimates, and they come with a good deal of uncertainty. (There’s debate over how much greenhouse gases can be released out of the Arctic, and how long it would take.) It all depends on how quickly the Arctic warms.

But the logic here is simple: The more warming, the greater the risk of kick-starting this feedback loop. A study published in Nature Climate Change in April predicted that 1.5 million square miles of permafrost would disappear with every additional 1°C of warming.

3) Ancient microbes are waking up

Last August, an outbreak of anthrax in Siberia sickened 72 people and took the life of a 12-year-old boy. Health officials pinpointed the outbreak to an unusual source.

Abnormally high temperatures had thawed the corpses of long-dead reindeer and other animals. Some of these bodies may have been infected with anthrax, and as Wired explained, the soil in Siberia is normally much too cold to dig deep graves. “The disease from thawing human and animal remains can get into groundwater that people then drink,” Wired reported.

Scientists are worried that as more permafrost thaws, especially in Siberia, there may be more outbreaks of long-dormant anthrax as burial grounds thaw.

That’s because the deep freeze of the permafrost doesn’t just keep carbon from escaping — it keeps microbes intact as well.

Permafrost is the place to preserve bacteria and viruses for hundreds of thousands — if not a million — years, explains Jean-Michel Claverie, a genomics researcher who studies ancient viruses and bacteria. “It is dark, it is cold, and it is also without oxygen. … There is no [ultraviolet] light.” All the bacteria need is a thaw to wake back up. “If you take a yogurt and put it in permafrost [that remains frozen], I’m sure in 10,000 years from now it still will be good to eat,” he said.

Claverie is part of a scientific team that recently determined it’s possible to revive 30,000-year-old viruses trapped in the permafrost. His work is centered on viruses that infect amoebas, not humans. But there’s no reason why a flu virus, smallpox, or some long-lost human infection couldn’t be revived the same way. These microbes are like time travelers — and they could thrive waking up in an age when humans have lost an immune defense against them.

I asked Claverie if there’s an upper limit to how long viruses and (certain types of) bacteria could survive in the permafrost.

“The limit is the limit given by the permafrost,” he explained, meaning he sees no limit. Permafrost is 1,000 meters deep in places, “which make it about a million, 1.5 million year old,” he said.

The danger here, he emphasized, is not from the slow thawing of the permafrost itself. That is, if the permafrost melts, and we leave the land alone, we’re unlikely to come into contact with ancient deadly diseases. The fear is that the thawing will encourage greater excavation in the Arctic. Mining and other excavation projects will become more appealing as the region grows warmer. And these projects can put workers into contact with some very, very old bugs.

The threat is tiny. But it exists. The big lesson is that even viruses thought to be eradicated from Earth — like smallpox — may still lurk frozen, somewhere.

“We could actually catch a disease from a Neanderthal’s remains,” Claverie says. “Which is amazing.”

4) Roadways are warping, foundations are shifting
When the permafrost melts, it literally changes the landscape.

“Oftentimes, permafrost has a great deal of ice in it, so you can get half the volume of that permafrost [that] may be frozen water,” Holmes says. “And so when that thaws, the water just runs off, you know? The water may head downhill or the water has a lower volume than is ice, so the ground just slumps and kind of falls apart.”

In Bethel, Alaska, roadways are literally rippling and warping as the ground beneath them becomes less solid. In other places, the melting permafrost is creating craters and sinkholes. “You see buildings that are kind of slumping into the ground; you see that a lot in the Russian Arctic,” Holmes says. Civil engineers are experimenting with new types of pilings and foundations to help keep Arctic buildings on strong footing.

There is one crater in Siberia so large it’s gotten the nickname “doorway to the underworld.” It’s a kilometer long and up to 100 meters deep. And it’s growing larger every year.

5) When we lose the permafrost, we lose a record of natural history

There are dangers buried in the permafrost. But there are also natural treasures yet to be discovered. The ice preserves all: ancient animal remains and human history in the region. Think of Ötzi, the remarkably preserved 5,000-year-corpse found in the Alps. If he had thawed, what was left of his body would have decomposed, and a window into the world he lived in would have been lost forever.

There may be other Ötzis in the Arctic. Or preserved bits of mammoth DNA yet to be discovered. The melting may make some of these treasures briefly accessible — freed from the ice — but also threatens to quickly destroy them. According to Scientific American, once a specimen is uncovered and thawed, researchers have a year at most to recover it before it completely breaks down.

Are there any benefits to melting permafrost?
“One story I heard of in Bethel is that there are people who are happy that now they can dig a basement,” Holmes says.

There are some benefits of thawing permafrost. For one, farming is now possible in parts of Alaska, as NPR reports. And “So it’s not that it’s all a bad news story,” Holmes says, “but I’d say the silver lining is pretty thin and pretty small in relationship to the larger-scale negatives.”