A ROW OF SKULLS glared from atop a wall of intricately stacked femurs and tibias. Though it was June, and she knew the sun was shining on the streets of Paris sixty feet above her, Dr. Maura Isles felt chilled as she walked down the dim passageway, its walls lined almost to the ceiling with human remains. She was familiar, even intimate, with death, and had confronted its face countless times on her autopsy table, but she was stunned by the scale of this display, by the sheer number of bones stored in this network of tunnels beneath the City of Light. The one-kilometer tour took her through only a small section of the catacombs. Off-limits to tourists were numerous side tunnels and bone-filled chambers, their dark mouths gaping seductively behind locked gates.