This book is about a 1994 expedition to explore the Sistema Huautla, a cave system in Mexico, which – at ~35 miles long and almost 5,000 ft. deep - is the deepest cave in the Americas, and the 5th deepest in the world (as of 2002, when the book was published.) Think of mountaineering expeditions – but underground. There were 44 people on this expedition, who carried massive amounts of equipment into the caves, while doing some pretty hairy rappelling in and around waterfalls. They also spent significant amounts of time route-finding through flooded tunnels using a technology Stone had invented to allow them to recycle their own breath (rather than hauling huge numbers of scuba tanks with them.) The one “camp” sounded like it was hammocks hanging off bolts in the cave walls above an underground river. (Not a good place to be in a flash flood!). Stone and Ende lived underground for 44 days during this expedition. This is not something that’s on my list to do - but the book was interesting and engaging.