Environmentalists are keeping the location of the island in the Sudd area secret to prevent poachers from killing the animals.

Sudan's north-south civil war caused massive displacement of animals as well as people into neighbouring countries, according to southern environment ministry official Victor Wurda la Tombe.

The conflict ended with a 2005 peace agreement that gave the south semi-autonomous status, but experts say game hunting is still unchecked in a region filled with guns despite a five-year ban on hunting to allow wildlife to replenish.

Environmentalists are only now beginning to discover the extent of the damage on animal populations, and are looking for additional pockets where animals could not be reached by rebels or armed groups looking for meat and export products like tusks.

It is possible there are other herds of elephants -- mostly unheard of in the contemporary south -- hiding out in the Sudd, an area so flat the Nile River breaks up into hundreds of channels and lakes.

"It's not that good a habitat for elephants, but they're free of people shooting at them," said Catterson on Sunday. "You and I wouldn't stand a chance in there between the mosquitoes and crocodiles. And you'd get lost."

Although Sudan is banned from exporting elephant tusks, it is still easy to purchase ivory carvings in Khartoum's famous Omdurman market. The Sudd is also host to a wide variety of fish, birds, amphibians and reptiles.

Two oil companies have been given concession rights by the southern government in areas deep in the Sudd previously undisturbed by seismic testing and exploratory drilling.

Both the ministry and international experts are worried about the potential for damage in the fragile swamp.