At the US Army Corps of Engineers’ research facility in central Alaska, a unique tunnel descends underground. Measuring over 350 feet deep, mammoth bones jut out from its surrounding walls. However, a team of researchers didn’t go to the remote site for ancient fossils. They were hunting for something much smaller—and smellier.

“The first thing you notice when you walk in there is that it smells really bad. It smells like a musty basement that’s been left to sit for way too long,” geological scientist Tristan Caro recounted in a statement. “To a microbiologist, that’s very exciting because interesting smells are often microbial.”

After adjusting to the rough scent, Caro and his colleagues focused on extracting samples of permafrost, each of which contained thousands of microscopic organisms. The microbes had spent as long as 40,000 years frozen inside of the icy soil, but after millennia of hibernation, it was time to wake up. What they did next would help researchers better understand—and possibly prepare—for what seems almost inevitable amid Earth’s warming temperatures.

“It’s one of the biggest unknowns in climate responses,” explained Colorado University Boulder (CU Boulder) geological scientist Sebastian Kopf. “How will the thawing of all this frozen ground, where we know there’s tons of carbon stored, affect the ecology of these regions and the rate of climate change?”

Microbes feast on organic matter, and subsequently release methane and carbon dioxide into the atmosphere. Unfortunately, these are two of the most potent and damaging greenhouse gases that humanity is trying to reduce. The prevailing concern is that thawing permafrost will release larger numbers of microbes, which will contribute to a vicious, closed loop of gas emissions.

“These are not dead samples by any means. They’re still very much capable of hosting robust life that can break down organic matter and release it as carbon dioxide,” said Caro.

To see what might happen as Arctic temperatures continue to rise, Caro and Kopf added water to the samples. In addition to breaking down organic matter, microbes also consume water to use for the fatty membranes that encase all living cells. They then incubated their Alaskan samples within a temperature range of 39 to 54 degrees Fahrenheit. That may not sound particularly warm, but it’s still above freezing—and extremely hot for the Arctic.

“We wanted to simulate what happens in an Alaskan summer, under future climate conditions where these temperatures reach deeper areas of the permafrost,” said Caro.

Their recent findings published in the journal JGR Biogeosciences documented an unexpected trajectory. Bacteria usually replace its entire colony’s cells within a matter of hours, but the permafrost samples began this process extremely slowly, sometimes replacing around one cell per 100,000 each day. Around six months later, however, their populations exploded. Some of the colonies grew so large that they created a viscous substance called biofilm that could be seen without a microscope.

At the same time, temperature appeared to play a smaller role in cellular growth than the researchers anticipated. Exposure to sudden, hotter temperatures didn’t speed up microbial reproduction all that much. The finding has major implications for the Arctic as its summers both grow warmer and longer.

“You might have a single hot day in the Alaskan summer, but what matters much more is the lengthening of the summer season to where these warm temperatures extend into the autumn and spring,” explained Caro.

It’s still unclear why the ancient microbes functioned like they did upon emerging from hibernation. Additionally, it remains to be seen if the slow start also occurs in other microbial colonies found around the world.

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“There’s so much permafrost in the world—in Alaska, Siberia and in other northern cold regions. We’ve only sampled one tiny slice of that,” said Caro.

There is at least one other major issue to consider, as well. While the study’s authors doubted their Alaskan microbe samples could infect humans, they played it safe by keeping them in a regulated, closed environment. Other species of bacteria that wake up from their long naps in the permafrost might be a bit crankier.