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 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 Nare in New Britain's Nakanai Mountains, where he first beheld the crushing beauty of river caves.