Blind descent / Nevada Barr.

Enhanced descriptions from Syndetics:

"Hands down, Anna Pigeon is the most complex protagonist in crime fiction," proclaimed the Philadelphia Inquirer of the heroine of last year's Endangered Species. "Barr possesses that rare combination of talents: she can write a beautiful sentence and she can create a first-rate mystery," said Publishers Weekly. In Blind Descent, Anna Pigeon faces personal demons as well as life-threatening dangers in an untamed underground wilderness for which neither training nor her love of the outdoors has prepared her.Lechuguilla Cavern is a man-eating cave discovered in New Mexico's Carlsbad Caverns National Park in the mid-1980s. Estimated to extend for more than three hundred miles, only ninety of them mapped, the cave was formed by acid burning away the limestone; corridors, pits, cramped wormholes, cliffs and splendid rooms the size of football fields tangle together in a maze shrouded in the utter darkness of the underground. When a fellow ranger is injured in a caving accident, Anna swallows her paralyzing fear of small spaces and descends into Lechuguilla to help a friend in need.Worse than the claustrophobia that haunts her are the signs-some natural, and some, more ominously, man-made-that not everyone is destined to emerge from this wondrous living tomb. All the skills Anna has honed in the terrestrial world are called into play on precipitous climbs, exhausting treks, and descents into canyons that have never seen the sun. The terrain is alien and hostile, the greed and destructive powers of mankind all too familiar. In this place of internal terrors, Anna must learn whom she can trust, and, in the end, decide who is to live and who is to die.