The orangutan is one of the most endangered great apes
A hitherto unknown population of orangutans numbering perhaps 1-2,000 has been found on the island of Borneo, conservation researchers say.
Members of the reclusive endangered species were found by scientists acting on tip-offs from local people.
Much of the orangutan's tropical forest habitat in Indonesia and Malaysia has been cut down for timber extraction and to create palm oil plantations.
About 50,000 orangutans are thought to remain in the wild.
"The reclusive red-haired primates were found in a rugged, largely inaccessible mountainous region," Erik Meijaard, of Nature Conservancy Indonesia, said.
The journey to the region took 10 hours by car, another five by boat and then a couple more hours hiking.
The team found more than 200 nests crammed into just a few kilometres and spotted three wild orangutans in the canopy above them - a mother and her baby, and a large male who broke off branches to throw at them.
It is even possible, the researchers say, that this could be a kind of orangutan refugee camp - with several groups moving into the same area following widespread forest fires.
The team of scientists is now working with local groups to try to protect the area.