We already knew that thawing Arctic permafrost would release powerful greenhouse gases. On Monday, scientists revealed it could also release massive amounts of mercury — a potent neurotoxin and serious threat to human health.

Permafrost, the Arctic’s frozen soil, acts as a massive ice trap that keeps carbon stuck in the ground and out of the atmosphere — where, if released as carbon dioxide, the greenhouse gas would drive global warming. But as humans warm the climate, they risk thawing that permafrost and releasing that carbon, with microbial organisms becoming more active and breaking down the ancient plant life that had previously been preserved in the frozen earth. That would further worsen global warming, further thawing the Arctic — and so on.

That cycle would be scary enough, but U.S. government scientists on Monday revealed that the permafrost also contains large volumes of mercury, a toxic element humans have already been pumping into the air by burning coal.

There are 32 million gallons worth of mercury, or the equivalent of 50 Olympic swimming pools, trapped in the permafrost, the scientists wrote in a study published in the journal Geophysical Research Letters. For context, that’s “twice as much mercury as the rest of all soils, the atmosphere, and ocean combined,” they wrote.

“As permafrost thaws in the future, some portion of this mercury will get released into the environment, with unknown impact to people and our food supplies,” said Kevin Schaefer, a scientist with the National Snow and Ice Data Center in Boulder, Colo., and a co-author of the study. The research was led by Paul Schuster, a scientist with the U.S. Geological Survey, and was co-authored by 16 other federal, university-based and independent researchers.

The scientists performed the research by taking cores from permafrost across Alaska. They measured mercury levels and then extrapolated to calculate how much mercury there is in permafrost across the globe, where it covers large portions of Canada, Russia and other northern countries.

“We figure that this represents the buildup of mercury during and since the last Ice Age,” Schaefer said.