The bog covers an area the size of England and is thought to contain billions of tonnes of peat.
Scientists say investigating the carbon-rich material could shed light on 10,000 years of environmental change in this little-studied region.
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.
Dr Simon Lewis from the Univer sity of leeds, who was working with PhD student Greta Dargie, 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.
The team estimates that the bog covers between 100,000 and 200,000 square kilometres (40,000 to 80,000 sq miles), with the peat-layer reaching up to 7m (23ft) beneath the ground. It spreads into the Democratic Republic of Congo.