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...