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.To the Supercave
There are several [effects of prolonged absolute darkness] that have been studied scientifically. One effect is that it disrupts normal circadian rhythms. 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.
Another thing is that each human brain has a unique tolerance for darkness. Some individuals reach their limit after a certain number of days or certain number of feet below the surface, and then they have an attack called The Rapture, which is like a panic attack on speed. I've interviewed people who've experienced it and they say it's like a panic attack but multiplied a hundred times in intensity.