“Watch your step.” Walker had stopped next to a giant electric substation, where a bundle of cables rose up and bypassed the beginning of an outlandishly large water main. Next to the water main was one final door, a green steel thing on rusty hinges, already propped open.

Standing with his wife and son, Mike peered through the doorway and into the tunnel. It was even longer than he’d expected, stretching straight ahead and angling slightly downward for what he guessed was two hundred yards before it came bending back up, so that anything beyond that was not visible. He didn’t like that. He liked it even less than the stench of stagnant, bacteria-smelling water breathing out from deep inside, as if they were standing in the gullet of a dying giant and looking down into its contaminated lungs. Across the ceiling, a single pipe the color of plumber’s caulk and the width of a good-sized oak tree ran down into the tunnel’s depths.

He touched Sarah’s hand. “Where did you hit Snow when you shot him?”

“In the chest.”

“Then what happened?”

“He fell down.”

“Dead?”

She just looked at him.

“I wasn’t there,” Mike said, and he could already tell they were sharing a thought, probably the same one running through Jolie’s mind and Walker’s as well. What if he’s still alive? What if the other end is locked?

Water dripped and oozed intermittently from the ceiling pipe, and the concrete floor beneath them was visibly damp. Far away Mike heard a soft pattering sound.

“It looks like about a quarter-mile from here to the far end,” Jolie’s voice said from up ahead, echoing down the length of the tunnel. “The old research facility is down at the other side.”

Mike linked hands with Sarah and Eli. Immediately to his left, he saw three bicycles leaned against the wall, old Schwinns from another era, the ones the maintenance crews had left there in the event they had to venture to the other end.

There was a sharp clang behind them.

Up ahead, Walker stopped and spun back in the direction they’d come. The green metal door they’d just passed through was now closed.

The stillness of the moment shattered around them, its components scattering everywhere. Mike heard noises growing louder up the tunnel, rusty scratching sounds, like rats scrambling down a metal pipe. It seemed to be coming from overhead. He felt a clammy tide of panic spill over him, tugging him back in its wake.

He looked up.