Here is a bit more on the article in the September 2006 National Geographic:
On an island off Papua New Guinea, a white-water river vanishes into a limestone cave. Following the torrent underground, a team discovers breathtaking waterfalls and theater-size chambers.
By Neil Shea
Photographs by Stephen L. Alvarez
An excerpt from the article in Nat Geo:
White water is fearsome enough on the surface. Pour a river into light-swallowing limestone tunnels and it becomes terrifying.
Deep beneath the rain forests of New Britain, an island off the coast of Papua New Guinea, churning rapids jet through enormous passages, some of the largest, most remote river caves on the planet. To reach them, explorers must first descend into massive dolines—sinkholes where soluble rock, weakened by runoff from an estimated 18 feet (5 meters) of rainfall a year, has collapsed. From the air they appear like impact craters, as if a volley of meteorites had long ago pummeled the forest.
"It's frightening when you see one—it's just a mass of white water at the bottom of a dangerous hole," says David Gill, a British caver.
An electrical engineer by trade, Gill taught himself to cave in the wet, cold potholes and the abandoned lead mines of Derbyshire in the English Midlands. Twenty-two years ago, he led a team to a doline called Naré in New Britain's Nakanai Mountains, where he first beheld the crushing beauty of river caves.
In January, Gill returned to the Nakanai, with 11 adventurers from the U.K., France, and the United States, on a two-month expedition to plumb one of the island's largest dolines, a half-mile-wide (800 meters) bowl called Ora.
The team's goal: To push deep into the cave at the bottom of Ora, map its enormous chambers, and follow the river boring through it—to the very end if possible.
"It's very, very remote," says Gill. "The terrain is so difficult. You can't hike in a straight line, and it's totally unexplored. Even the local people don't go up there. There's nothing there for them."
From Port Moresby, capital of Papua New Guinea, the men traveled by plane and boat to Matong, a shore camp for loggers on New Britain. Then lumber trucks hauled them to another camp, where the roads disappeared. A helicopter dropped them at a small settlement inhabited by a hundred members of the Kol people and two families of missionaries from the U.S. and Australia.
At first, villagers suspected the outsiders were gold hunters. A few even pulled team members aside to show off the yellow lumps they'd found. "We'd just say, 'Oh yeah, fool's gold,' " says Dave Nixon, 38. "It's hard to explain that gold deposits don't occur in limestone."
The community soon warmed, and most villagers agreed to work as porters hauling supplies to base camp, a three-hour trek to a ridge overlooking the Ora Doline. Then the rain began, weeks of it, transforming the forest into a gleaming, mud-slick obstacle course.
At the bottom of the doline, the explorers followed the river into one side of the cave, then the other, hugging the narrow riverbanks underground, the water rumbling like a freight train. Often the banks disappeared, forcing the men to cross the river using ropes—a dangerous traverse where one caver would swim across, water boiling over him, to fix a line for the others.
Jean-Paul Sounier volunteered for most of the swimming. Sounier, 55, has been caving for 40 years and made five previous pilgrimages to caves below the Nakanai. "You can't afford an accident," he says. "It's not like home where, if you have an injury, a rescue team will be quick to get you out." On New Britain, there was no rescue team.
www7.nationalgeographic.com/ngm/0609/feature3/index.html
