Then, just a week into a research trip, a team from Stockholm University found “vast methane plumes” shooting from the sea floor off the Siberian coast. Columns of gas bubbles, they reported, were surfacing around their icebreaker in waters saturated with 10 to 50 times more methane than usual.

This was the marine equivalent of melting permafrost, the undoing of frozen crystals called methane hydrates, locked solid for millennia by the pressure and temperature of deep oceans.

The U.S. Office of Naval Research calculates that methane hydrates hold trillions of tons of hydrocarbons, from two to 10 times the amount as all conventional deposits of fossil fuels, but they’re probably too costly or unsafe to harvest. Now, as ocean temperatures rise, they’ve begun collapsing, spewing as much gas skyward as the thawing tundra.