First, a quibble. As near as I can tell, the book is about a competition to explore the deepest cave, which is defined in terms of the cave, rather than with respect to sea level. From one perspective, this is the opposite of mountaineering definitions, in that Everest is the tallest not because of its height from base to summit, but because of the height of its summit with respect to sea level. From another perspective, however, it shares with the mountaineering definition a sort of dodge from what someone unfamiliar with the field might expect: the tallest mountain isn't as tall as other mountains and similarly, the deepest place on Earth is above sea level.
Second, there were points in the book where I just could not get past confusion in the text. The most notable example of this is the description of one of the Bill Stone expeditions where they were digging around in a new sinkhole and simultaneously exploring related caves in the area. Upstream and downstream got reversed at least once. (I apologize for the lack of page numbers; I read a borrowed copy and returned it before writing the review.)
Third, and most important, I felt like some of the breathless "last frontier" stuff was utter crap. I'm sure that at least some of the cavers involved buy into this, but it is abundantly clear that most of this community isn't going to go take up some other hobby just because The Big One has been bagged. Also, there are so many other areas left to investigate that it's more than a little silly. Like, the _actual_ deepest places on Earth, which will surely be way, way, way below sea level.
Tabor treats the competition to identify the "deepest" cave (greatest vertical difference between top and bottom) as a race between an American team led by Bill Stone, exploring a variety of Mexican Supercaves, notably Cheve, and some Ukrainians led by Alexander Klimchouk, exploring caves on the Arabika Massif. The strategy in both cases was similar: find a tall bit of limestone and look for a cave that starts near the top and goes all the way to the bottom. Both did dye experiments (among other things) to justify further exploration of caves that previous explorers had dismissed.
Tabor provides some biographical detail on Stone, Klimchouk, and a variety of other cavers and divers. He spends a lot of time on the rebreather technology developed by Stone. There's some mildly juicy gossipy bits about people having sex in caves, some of which has some really moralistic overtones. Meh. I don't disagree with the moralizing for the most part.
While staying close to the story, Tabor does take the opportunity along the way to comment on the difference in leadership styles, expedition organization, and so forth. The topic lends itself to this well -- Tabor could have done a lot more, and it would be fairly easy to imagine a business or organizational psychology case study that compared and contrasted the two groups. The stylistic differences show up in every conceivable area (how the groups handled food, who led, number of accidents/fatalities, risks taken, percentage of women participants, and on and on and on). While it would be possible to chalk any one or a few of these differences up to random or arbitrary or unrelated causes, collectively they dovetail very nicely with the personalities of the principals as described by Tabor.
It's an interesting story, a quick read, and accessible on many levels. Not a waste of anyone's time.
Also, it's kind of fun to watch while Tabor kicks Outside magazine around (a tremendously worthy target).
ETA: Forgot to mention. The second fatality of the book was the scuba diver using the rebreather technology. A scuba diver with insulin dependent diabetes. I'm just not even sure where to start on that, but how about, who the hell let him in the _cave_ never mind doing half the diving exploration with the brand new technology? Followed up with some icing of am Ende diving with the rig the guy died using -- after lying about who was doing the diving because am Ende and Stone knew the sherpas would balk if they knew who was doing the diving. The list of Bad Decisions on Stone expeditions is basically endless.