Permafrost is stranger than it sounds.

Beneath roughly a quarter of the Northern Hemisphere’s land lies ground that has been frozen for thousands of years — in some places, for hundreds of thousands. It’s called permafrost: soil, rock, and sediment held at or below freezing for at least two consecutive years, though most of it has been frozen since the last ice age. And locked inside that frozen ground is roughly twice as much carbon as currently floats in the entire atmosphere. As the Arctic warms, the freezer is starting to thaw — and what’s inside is waking up.

Permafrost is not just “frozen dirt.” In the high Arctic it can extend hundreds of meters deep, a permanent foundation that has shaped everything above it: the polygonal patterned ground, the pingos (ice-cored hills), the shallow summer ponds. Only the top layer — the “active layer,” a meter or so — thaws each summer and refreezes each winter. Everything below stays frozen, year after year, century after century. Buildings, roads, and pipelines across Alaska, Canada, and Siberia are engineered around it; the Trans-Alaska Pipeline is elevated on supports specifically so its heat won’t melt the ground it crosses.

The carbon got there the slow way. Over tens of thousands of years, dead plants and animals in the cold north decayed only partially — the cold slowed decomposition to a crawl — and layer after layer of half-rotted organic matter accumulated and froze into the ground. The result is a carbon vault on a staggering scale: an estimated 1,500 billion tons, about double what’s in the atmosphere today. As long as it stays frozen, it’s inert. Thaw it, and microbes wake up and finish the decomposition their ancestors started — releasing carbon dioxide, and where water pools in the thawed ground, methane, a far more potent greenhouse gas.

The thaw is already visible. Across the Arctic, permafrost is warming and the active layer is deepening. The ground collapses into thermokarst — pittted, sunken landscapes where ice wedges have melted out. Coastlines held together by frozen ground are crumbling into the sea. In Siberia, thawing permafrost has buckled roads, tilted houses, and in 2020 contributed to a catastrophic diesel spill at Norilsk when a fuel tank’s foundations gave way. Ancient things are emerging too: perfectly preserved mammoths, and — more worryingly — ancient microbes, revived after tens of thousands of years frozen.

Scientists call the danger a feedback loop, and it’s the part that keeps them up at night. Warming thaws permafrost; thawing releases greenhouse gases; greenhouse gases cause more warming. The vault doesn’t need to empty all at once to matter — a steady leak, century after century, adds a significant extra push to climate change on top of human emissions. Current estimates suggest permafrost could release tens to hundreds of billions of tons of carbon this century if warming continues, much of it as methane from waterlogged thaw.

Permafrost is the planet’s deep freezer — and like any freezer, what matters isn’t just what’s inside, but whether the door stays shut. For ten thousand years it did. Now the seal is cracking, and the coldest ground on Earth is telling us, in collapsing hillsides and bubbling lakes, exactly how much was stored in the dark.