Krubera is like a gigantic elevator shaft. Ninety percent of its descent terrain is technical, meaning the cavers are on rope descending or ascending. At 7,188 feet, you could stack seven Empire State Buildings within the depth that Krubera represents. It's also murderously cold. The water is never much more than 32 degrees and the air temperature in summer is 34-36 degrees. Many expeditions prefer to climb there in winter because there is less chance of flash flooding, but then the air temperature is about zero degrees Fahrenheit and the wind chill remains below zero.

Cavers not only have to contend with the climbs and the extreme verticality, they have to deal with constant absolute darkness. Unless they are moving or performing a task they turn off their lights to save battery power, so most of the time they are in the dark. They are always wet and cold and there is always a high level of anxiety. They typically lose a pound or a pound-and-a-half a day, in part because of the kind of physical work that is required -- descending or rappelling with very heavy loads, and ascending the same way. In Krubera, cavers are underground for up to a month.

Cavers may work for twenty-four hours at a stretch and then sleep for twenty or twenty-four hours. Second, their immune systems really take a beating without sunlight or natural light. Stone told me that after he had been underground in Cheve for two weeks, every one of his fingernails became infected with staphyloccocus. [...]

There is so much vertical work in caving that you cannot use lightweight rappel hardware like mountain climbers do. The first big development came in the 1960s, when a John Cole invented what cavers now call the rappel rack -- essentially a supersized, beefed up rappelling device that has enabled cavers to do seven-hundred-foot free vertical drops on slick, muddy ropes, and still have control.

But by the late 1980s cavers had reached the limits of traditional open circuit scuba equipment in terms of deep caves, because there were only so many of those forty-five pound tanks that one could carry down. So Stone decided to create a rebreather, which allows a diver to re-breathe his or her own breath. [...] Stone remained underwater for twenty-four hours with a prototype, and the only problem he had was staying awake.