When Mesa Verde National Park dispatcher Frieda Dierkz, on an avocational expedition to explore and survey the Lechuguilla cave in New Mexico's Carlsbad Caverns, is trapped 800 feet beneath the earth's surface by a head injury and a shattered leg, the person she asks authorities above to send after her is her friend Anna Pigeon. Following a week's worth of deep breaths, Anna, together with Carlsbad cave specialist Oscar Iverson and Underground Resource Coordinator Holden Tillman, undertakes a nine-hour journey she compares to "an expedition into outer space" toward Frieda and the five other members of her crew-- only to hear from Frieda that her accident was no accident at all. Before the rescuers can return with Frieda to the surface, another disastrous "accident" heightens the mystery. Then the grueling tour de force of the novel's subterranean first half is matched by violence aboveground as well, and by unwelcome revelations suggesting that several of Frieda's companions--a former lover, his jealous wife, a veteran caver whose sister was killed on Frieda's watch--may have had good reason to kill her. With all the irresistible force of nightmare, Anna's pulled back on a return visit to Lechuguilla, where she'll find much more than she bargained for. Barr's superbly unerring eye for natural setting and human conflict has made Anna's five earlier adventures (Endangered Species, 1997, etc.) as distinctively memorable as the National Parks themselves. This installment is the most suspenseful of all, even though claustrophobes are well-advised to stock up on Prozac before turning the first page.