In 2004, two great scientist-explorers attempted to find the bottom of the world. American Bill Stone took on the vast, deadly Cheve Cave in southern Mexico. Ukrainian Alexander Klimchouk targeted Krubera, a freezing nightmare of a supercave in the war-torn former Soviet republic of Georgia. Both men spent months almost two vertical miles deep, contending with thousand-foot drops, raging whitewater rivers, monstrous waterfalls, mile-long belly crawls, and the psychological horrors produced by we...