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. Here were the remains of six million Parisians who had once felt the sun on their faces, who had hungered and thirsted and loved, who had felt the beating of their own hearts in their chests, the rush of air in and out of their lungs. They could never have imagined that one day their bones would be unearthed from their cemetery resting places, and moved to this grim ossuary beneath the city.

That one day they would be on display, to be gawked at by hordes of tourists.

A century and a half ago, to make room for the steady influx of dead into Paris ’s overcrowded cemeteries, the bones had been disinterred and moved into the vast honeycomb of ancient limestone quarries that lay deep beneath the city. The workmen who’d transferred the bones had not carelessly tossed them into piles, but had performed their macabre task with flair, meticulously stacking them to form whimsical designs. Like fussy stonemasons, they had built high walls decorated with alternating layers of skulls and long bones, turning decay into an artistic statement. And they had hung plaques engraved with grim quotations, reminders to all who walked these passageways that Death spares no one.