Overton took out his tablet and studied the blueprint. “All right, looks like we’re somewhere under the spaceport,” he said, pointing to a horizontal line on the display. “Alexia puts the shuttle’s possible location here. Which means we’re going to have to go above ground to get to the hangar.”

He looked at the mission clock on his HUD.

12:45 p.m.

“We could wait until it gets dark, but I’m not sure it’ll make much difference.”

Sophie shook her head. “Not a chance I’m staying down here for that long.”

Overton grabbed his helmet and hoisted himself off the ground by grabbing onto a pipe, nearly ripping it free from the wall. “Roger that. Let’s keep moving.”

They picked up their rifles and continued into the darkness, pushing along the winding tunnels, up stairways, and through countless corridors. Several signs hung from the walls. They were rusted beyond recognition by the steady drip from the leaky pipes lining the ceiling.

Sophie finally knew what it was like to be one of the mice she had seen used in countless experiments. Navigating the tunnels was like being stuck in a maze—no, a rat trap. Even with the veteran recon Marine and the world’s smartest computer as her guides, she was starting to feel lost.

As they rounded a corner, she saw the passage opened up into another vestibule. Her trot turned into a steady jog, and soon she was leading Overton up the stairs into a massive room.

This was different from the vestibule they had entered right off the train. It was larger, with a curved dome, much like Biome 1. The walls were all concrete, with painted arrows pointing at different passages. With the night vision, she couldn’t make out the different colors, but she could read them.

HOLLOMAN AIR FORCE BASE 21 MILES

WHITE SANDS TEST FACILITY 14 MILES

SPACEPORT 2¼ MILES

WHITE SANDS POST HEADQUARTERS 13 MILES

“Like Alexia said, this was the biggest military installation in the United States at one time,” Sophie said, preempting any snide comments from Overton regarding the distances.

“We’re halfway there. Let’s keep moving.”

They hurried into the tunnel labeled “Spaceport.” It was much wider—about three times the diameter of the others, large enough for a vehicle to travel in.

“This tunnel should open up into one last vestibule that leads to the surface,” Alexia said in their earpieces.

“And you really think Secundo Casu is going to be waiting for us?” Overton asked.

Silence washed over the com. Sophie had stopped several paces ahead, her figure frozen in the darkness like a statue.

“What is it, Sophie?” Overton continued.

“I think I hear something . . .”

Overton halted and strained his ears. Nothing but the sporadic burst of static over the com filled his helmet.

“I don’t—”