Cave diving is the most psychologically challenging and physically dangerous "adventure sport": a twisting, claustrophobic journey down submerged limestone tunnels no wider at times than the body, opening into lightless caverns as desolate and beautiful as the dark side of the moon. In this fourth thriller featuring black-sheep salvage diver and ex-SEAL Lyle "Tiller" Galloway, David Poyer has written some of the most exciting underwater action scenes in recent fiction. Tiller is broke and desperate when he gets a call for help. A Vietnam pal, Bud Kusczk, has died in a diving accident. His widow, Monica, needs Tiller's savvy to temporarily manage Bud's scuba business in Florida. At the same time, Tiller's estranged son, Tad -- who has taken after his father in running from the law -- shows up at his door. Together they leave Cape Hatteras for north Florida, and the submerged caves in which Bud drowned. Tiller is intrigued by these mysterious caves, and tempted too by alluring Monica Kusczk. But Monica can't explain how Bud's business produced such huge profits. At the same time, Tad discovers danger above ground, when he spies on a group of powerful men who seem well stocked with cash, drugs, and automatic weapons. As Tiller learns more about the mysterious world of the caves, and their link to Florida's vanishing aquifers, he begins to suspect that his friend's death was no accident -- and that the truth lies hidden somewhere deep beneath the ground, in airless and bewildering passages that claimed the life of his friend.