“Yes, yes.” Which came out more as a breath than words.

She pulled a halogen flashlight from her pocket, a tiny but powerful thousand-lumen model. A Fenix PD35.

Ercole squinted, surely thinking: Rubber bands, flashlight, flick-blade knife? These Americans certainly came prepared.

A nod toward the door.

His Adam’s apple bobbed.

She pushed inside, raising the light and the gun.

There was a startling crash; the door had struck a table, spilling a large bottle of San Pellegrino mineral water.

“He’s here!” Ercole whispered.

“Not necessarily. But assume he is. He may have set up the table to warn him somebody breached. We have to go fast.”

The entryway atmosphere was pungent, the walls covered with graffiti. It resembled a cave in some wilderness, rather than a man-made structure. A stairway led down two flights. They went slowly. The halogen would give them away but it was their only source of illumination. A fall down these steep stones could be fatal.

“Listen,” she said, pausing at the bottom. She believed she’d heard a moan or grunt. But then nothing.

They found themselves in an old brick tunnel about eight feet wide. The aqueduct, a square-bottomed trough about two feet across, ran through the middle. It was largely dry, though old iron pipes overhead—the ceiling was six feet above them—dripped water.

Ercole pointed to their left. “The reservoir would be there, if the map is correct.”

A rumbling began in the distance and grew in volume. The floor shook. Sachs supposed that it was the subway, nearby, she recalled from the map, but it also occurred to her that Naples was not so very far from Mount Vesuvius, whose volcano she’d read might erupt at any time. Volcanoes equal earthquakes, even the smallest of which might pin her under rubble—and leaving her to die the worst death imaginable. Claustrophobia was her big fear.

But the roaring rose to a crescendo, then faded.

Subway. Okay.

They arrived at a fork, the tunnel splitting into three branches, each with its own aqueduct.

“Where?”

“I am sorry. I do not know. This part was not on the map.”

Pick one, she thought.

And then she saw that the left branch of the tunnel contained not only an aqueduct but a terracotta pipe, largely broken. Probably an old sewer drain. She was recalling the scatological trace from the Composer’s shoes. “This way.” She began along the damp floor, the smell of mold tickling her throat and reminding her of the uranium-processing factory in Brooklyn, site of the Composer’s first murder attempt.

Where are you? she thought to the victim. Where?

They pressed on, walking carefully in the aqueduct until the tunnel ended—in a large, dingy basement, lit dimly from airshafts and from fissures in the ceiling. The aqueduct continued on arched columns to a round stone cylindrical structure, twenty feet across, twenty high. There was no ceiling. A door had been cut into the side.

“That’s it,” Ercole whispered. “The reservoir.”

They climbed off the aqueduct and down stone stairs to the floor, about ten feet below.