It's the story of two elite cavers - American Bill Stone and Ukrainian Alexander Klimchouk who were each competitively searching for the world's deepest cave.
Stone placed his bets on Chevé, a cave in the Sierra Juárez mountains of Oaxaca, Mexico. Chevé is almost, but not quite, 5,000 feet deep making it the deepest cave in the Americas. But it sits in a 9,000 foot thick limestone plateau and fluorescein dye tracing shows it has to potential to be as deeper than 8,000 feet (the dye emerged 13 miles away at the base of the plateau). The trick is linking up Chevé with caves at the base of the limestone plateau - it's finding your way through a three-dimensional maze.
The worst obstacle confronting these cavers are sumps. Water filled passages that require specialized scuba equipment to pass through. It's bad enough to get hurt deep in a supercave like Chevé where it may be impossible to get you back to the surface (and certainly no quicker than days later), but get in trouble in a sump with scuba gear and you will die an unpleasant death (as did another young caver mentioned in the book).
While some sumps had cave beyond that continued, Chevé ended in a terminal sump (one that went no further). There are, however, miles of passages yet to explore that may well lead deeper and cavers like Bill Stone obsessively return year after year to find and map them.
The Ukrainian caver Alexander Klimchouk placed his bets on Krubera, located on the Arabika Massif in the Western Caucuses of Georgia. He won - Krubera is officially the deepest cave on Earth (the only cave deeper than 2,000 meters) at 2,191 ± 20 m (7,188 ± 66 ft). Krubera is described as a nasty cave with many long, deep drops interspersed with muddy, meandering, tight crawls between them. It's also very cold and very wet.
When reading the book, it does seem a bit over melodramatic at times repeatedly reiterating all the dangers faced by these extreme cavers and how physically and mentally fit they have to be to survive the stresses encountered.
Then, sitting comfortably on your couch at home, you try to visualize being 2,000 feet below the surface of the Earth, 2 miles and 20+ climbs up 100+ foot pits from the cave entrance, in water up to your neck preparing to go under and explore a flooded cave passage where no one on Earth has been before, and where one little problem can spiral out of control leading to a painful, horrifying death, you realize that these guys (and a few women) are completely fucking nuts.
I would definitely suffer what Tabor calls the "Rapture", compared to being like a panic attack on meth, that sometimes strikes cavers who think too hard about where they are and what they're doing.
The modest entrance to Krubera, the deepest cave on Earth