In a connecting cave called Phantom Pot, the explorers crawled through a tight, sinuous passage lined with skin-ripping rock. Each journey into Phantom required a four-hour ordeal in the cheese grater—two hours each way. Then, back at the surface, the men slogged uphill to camp, sometimes in rain, sometimes in darkness, achorus of frogs and insects whirring in the trees.

"We're quite masochistic," says Dave Nixon, laughing. "But it's character-building stuff."

In the end, the team discovered nearly eight miles of river caves. Gill hopes the expedition's work will help persuade the Papua New Guinea government to create a conservation area protecting the Nakanai Mountains. The Malaysian government did something similar on Borneo—with instrumental guidance from Gill—at Gunung Buda, another region of giant caves, which was declared a national park in 2001.

"The thrill of exploring where nobody has ever been before, of being the first light to ever shine in the darkness . . . it's awe-inspiring," Gill says. "The world of caves remains relatively untouched. It is the ultimate adventure."