To climb straight back to the surface, without stopping to rig ropes and phone wire, would take them four days. It took three days to get back from the moon…

Casteret and Chevalier helped turn caving into a heroic undertaking, and the search for the world’s deepest cave into an international competition—a precursor to the space race…

“In the past, I’d lose twenty-five pounds on one of these trips,” Stone told me. “We can burn as many calories as a Tour de France rider every day underground.” Ascending Chevé, he once said, was like climbing Yosemite’s El Capitan at night through a freezing waterfall. To fine-tune the team’s diet, he’d modelled it on Lance Armstrong’s program, aiming for a ratio of seventeen per cent protein, sixteen per cent fat, and sixty-seven per cent carbohydrates…

Over the years, caving gear has undergone a brutal Darwinian selection, lopping off redundant parts and vestigial limbs. Toothbrushes have lost their handles, forks a tine or two, packs their adjustable straps. Underwear is worn for weeks on end, the bacteria kept back by antibiotic silver and copper threads. Simple items are often best: Nalgene bottles, waterproof and unbreakable, have replaced all manner of fancier containers; cavers even stuff their sleeping bags into them. Yet the biggest weight savings have come from more sophisticated gear. Stone has a Ph.D. in structural engineering from the University of Texas and spent twenty-four years at the National Institute of Standards and Technology, in Gaithersburg, Maryland. His company has worked on numerous robotics projects for NASA, including autonomous submarines destined for Europa, Jupiter’s sixth moon. The rebreathers for the Chevé trip were of his own design. Their carbon-fibre tanks weighed a fourth of what conventional tanks weigh and lasted more than four times longer underwater; their software could precisely regulate the mix and flow of gases.

Stone’s newest obsession was a set of methanol fuel cells from a company called SFC Energy. Headlamps, phones, scuba computers, and hammer drills (used to drive rope anchors into the rock) all use lithium batteries that have to be recharged. On this trip the cavers would also be carrying GoPro video cameras for a documentary that would be shown on the Discovery Channel. In the past, Stone had tried installing a paddle wheel underground to generate electricity from the stream flow, with fairly feeble results. But a single bottle of methanol and four fuel cells—each about the size of a large toaster—could power the whole expedition…

“Welcome to Hell,” one of the cavers told me, when I joined him by the campfire that first night. “Where happiness goes to die,” another added. There was a pause, then someone launched into the colonel’s monologue from “Avatar”: “Out there, beyond that fence, every living thing that crawls, flies, or squats in the mud wants to kill you and eat your eyes for jujubes. . . . If you wish to survive, you need to cultivate a strong mental attitude.” It was a favorite conceit around camp: the cloud forest as hostile planet…

“Where did the water go a million years ago? That’s what you have to ask yourself,” Stone said. “As a cave diver, you have to think four-dimensionally.”