Scientists have confirmed the existence of a tropical peatland about the size of England in a remote part of the Republic of Congo.
They say the partially decayed organic matter could throw light on 10,000 years of environmental change.
The enormous tropical peatland was first spotted by satellites, but an expedition carried out in April this year revealed its immense scale, the BBC reports.
The discovery team, from the University of Leeds, the Wildlife Conservation Society-Congo and Congo-Brazzaville's Marien Ngouabi University, had to contend with dwarf crocodiles, gorillas and elephants as they explored the area. But they said the biggest challenge was soggy feet.
The bog is between 100,000 and 200,000 square kilometres in size, with the peat layer reaching seven metres down.
Scientists estimate it holds billions of tonnes of partially decayed vegetation, which means it's a vast store of carbon.
Team member Dr Simon Lewis said: "You can only walk on these areas for a couple of months a year, right at the end of the dry season, so you have to time it right. Even then it is still wet every day.
"We were inside the swamp for three weeks, and the only time we had dry feet was when we were asleep in our tents. To place the tent, you have to build a platform because the ground is permanently water-logged."