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Rising temperatures are waking a sleeping giant in the North—the permafrost—and scientists have identified a new danger that comes with that: massive stores of mercury, a powerful neurotoxin, that have been locked in the frozen ground for tens of thousands of years.

The Arctic’s frozen permafrost holds some 15 million gallons of mercury. The region has nearly twice as much mercury as all other soils, the ocean and the atmosphere combined, according to a new study published Monday in the journal Geophysical Research Letters.

That’s significantly more than previously known, and it carries risks for humans and wildlife.

“It really blew us away,” said Paul Schuster, a hydrologist at the U.S. Geological Survey in Boulder, Colorado, and lead author of the study.

Mercury (which is both a naturally occurring element and is produced by the burning of fossil fuels) is trapped in the permafrost, a frozen layer of earth that contains thousands of years worth of organic carbon, like plants and animal carcasses. As temperatures climb and that ground thaws, what has been frozen within it begins to decompose, releasing gases like methane and carbon dioxide, as well as other long dormant things like anthrax, ancient bacteria and viruses—and mercury.

“The mercury that ends up being released as a result of the thaw will make its way up into the atmosphere or through the fluvial systems via rivers and streams and wetlands and lakes and even groundwater,” said Schuster. “Sooner or later, all the water on land ends up in the ocean.”

Mercury Carries Serious Health Risks

Though the study focused on the magnitude of mercury in the North, Schuster said that’s just half the story. “The other half is: ‘How does it get into the food web?’” he said.

Mercury is a bioaccumulator, meaning that, up the food chain, species absorb higher and higher concentrations. That could be particularly dangerous for native people in the Arctic who hunt and fish for their food.

Exposure to even small amounts of mercury can cause serious health effects and poses particular risks to human development.

“Food sources are important to the spiritual and cultural health of the natives, so this study has major health and economic implications for this region of the world,” said Edda Mutter, science director for the Yukon River Inter-Tribal Watershed Council.

This Problem Won’t Stay in the Arctic

The mercury risk won’t be isolated in the Arctic either. Once in the ocean, Schuster said, it’s possible that fisheries around the world could eventually see spikes in mercury content. He plans to seek to a better understand of this and other impacts from the mercury in subsequent studies.

The permafrost in parts of the Arctic is already starting to thaw. The Arctic Council reported last year that the permafrost temperature had risen by .5 degrees Celsius in just the last decade. If emissions continue at their current rate, two-thirds of the Northern Hemisphere’s near-surface permafrost could thaw by 2080.

The new study is the first to quantify just how much mercury is in the permafrost. Schuster and his co-authors relied on 13 permafrost soil cores, which they extracted from across Alaska between 2004 and 2012. They also compiled 11,000 measurements of mercury in soil from other studies to calculate total mercury across the Northern Hemisphere.