Narrow passages, twisting upward or dropping precipitously. Huge vaults filled with fantastic shapes. Tunnels twined in tangled mazes. Over centuries, underground rivers can carve holes and rooms in solid rock; drips of water build walls of stone. Natural caves shape another world beneath our feet. Dangerous and beautiful, these places remain unknown--until someone decides to investigate. In 2004, businessman and caver John Ackerman drilled an entryway into Goliath Cave, a huge and unexplored complex in the karst region of southeastern Minnesota. Squeezing through tiny openings, scuba diving through silt-filled waters, scaling walls, and traversing crevasses, he and his fellow cavers painstakingly mapped ever-further reaches of the complex in an exploration that continues to this day. But man-made caves that do not breathe can be even more dangerous than their natural cousins. In St. Paul, also in 2004, five teenagers entered an area where intermittent fires robbed the air of oxygen. Only two emerged alive.