Central Africa's peatlands hold gigatons of carbon - a stockpile that poses a grave threat to hopes of limiting global warming to 2°C.
The product of vegetation decay that occurred aeons ago, the carbon has been safely locked in the soil for thousands of years, but risks being unstoppered by farming.
Released into the air, the gas could add dramatically to greenhouse-gas emissions caused by fossil fuels.
"We have a map of the central Congo peatland we published for the first time this year, which shows that they cover around 145,000km², an area a bit bigger than the size of England," said Simon Lewis, a scientist from Britain's University of Leeds, on a soil-sampling mission to remote northwest DR Congo.
"We think it stores about 30billion tons of carbon. That's as much carbon as the emissions from fossil fuels, all the emissions from humanity globally for three years."
For nearly two decades, climate scientists have warned of the threat of positive feedbacks - a vicious circle of global warming.
Fears have focused primarily on the potent greenhouse gas methane seeping from thawing Arctic permafrost. These emissions would add significantly to warming, which would thaw more permafrost - and which in turn would release more greenhouse gas to stoke global temperatures, and so on.
But the dark, swampy peatlands of the tropics are now also a major area of concern.
This has made draining the soil for farming and slash-and-burn agriculture big climate threats.
In 2015, the World Resources Institute calculated that fires in Southeast Asia, where much land has been converted for palm oil and other products, spewed more greenhouse gases into the air than all US economic activity in 26 out of 44 monitored days.
The Congo basin is exceptionally rich in peat - about 2m thick, according to a study published in the science journal Nature in January by Lewis and colleagues.
So far, the peat remains largely undisturbed. Campaign groups are desperate for it to remain so, and for the forests which suck carbon dioxide from the air to be preserved.