She’d been down here in the early eighteenth century with—which one—was it Lollie? Yes, poor Lollia, she’d been on her last legs then, frantically trying on powdered wigs and loading her face with leaded creams. No use, any of it.

Now another turn—and there was the great cross. But would the entrance still be behind it? Had the French found it and walled it up, also? She looked at the cross, refocusing her vision so that the glyphs that were invisible to the human eye stood out clearly. There was a word to utter in Prime, according to the instructions, that would open the wall. It would also alert the Keepers inside that she was there. She was instructed, as well, to append her name. She drew a deep breath, expanded her lungs, and released the call. It vibrated against the stone, its echo fluttering off down the corridors. Now, the instructions told her, press thrice the upper corner of the left tine of the cross.

The hidden door swung inward. Beyond was absolute dark. From within there came the odor of many lairs.

“Halte!” Everybody froze. “Do not move,” Colonel Bocage said. “Listen.”

It came again, a sound so deep that it was more a sensation than a noise.

“Could it be the Métro?” Becky asked. “Are we under a tunnel?”

Des Roches laid his arm around her neck, drew her close, uttered a breathy, almost-silent whisper. “That was a vampire.”

The French were in some ways far advanced in their knowledge. Becky and Charlie and Paul had been down many a dark tunnel, but they had never had to face anything this elaborate.

The place stank of wet stone and the musk of bats. The group had stopped its march and was clustering together in a low gallery. Becky shone her light on a stone where graffiti had been carved. “Merci à Dieu, m—” and there it stopped, as if the carver had run out of strength or light or time. The letters were formed in an old script, perhaps from the last century or the one before.

“Listen—again,” Charlie said.

It was eerie, that sound—so deep that you wouldn’t notice it unless you were trained to. Paul had heard it echoing off the walls of the night in Beijing, in Osaka, in Bangkok. As a boy, he had heard it mixing with the sullen hoot of the owl and the barking of the fox.

After some moments, there was no sound but their breathing and the busy scuttle of rats. The roar of the great city above them was a bare whisper. Water dripped, echoing as if somewhere deeper there might be a hidden pool or even a small lake.

“Lights out,” Paul said. “We go to light-amplification goggles.” Their equipment packs rattled as they put the night-vision goggles on.