A recent survey of Arctic permafrost has revealed that the northern tundra holds the world’s largest reserve of mercury, with its size estimated to be in the tens of millions of gallons. Ordinarily, the presence of the toxic metal wouldn’t be a problem, as it is locked away in the frozen soil, but researchers are concerned that as the permafrost melts due to global warming, that mercury trapped there could be released into the environment.

Many naturally-occurring elements circulate through the Earth’s soil, eventually carried by groundwater to sources of open water, evaporated into the atmosphere and then re-deposited onto the soil via rainfall. But in regions where the ground is frozen year-round, many of these elements wind up trapped there, including the accumulation of elements such as mercury.

This new survey, using permafrost core samples from 13 sites across Alaska, estimates that there is 58 million liters (15 million gallons) of this heavy metal trapped in permafrost across the Arctic. If this estimate is correct, that would make the Arctic the world’s largest reserve of mercury, approximately double the amount of what the rest of the world combined is estimated to hold.

And the Arctic has been heating up faster than the rest of the planet, melting permafrost that has been in place for millennia, prompting fears that various elements trapped there–greenhouse gases such as carbon dioxide and methane, or dormant pathogens like anthrax and other long-dormant ancient diseases–could be released into the environment.

"There would be no environmental problem if everything remained frozen, but we know the Earth is getting warmer," warns study author Paul Schuster, a hydrologist at the U.S. Geological Survey in Boulder, Colorado. "This discovery is a game-changer."