She squeezed her eyes shut and tried to relax, extending the time between inhalations and forcing a steady breathing rhythm. When she felt her heartbeat slow, she pried open her eyes, but still found herself surrounded by a cloak of blackness. Never prone to claustrophobia, she still couldn’t help feeling like she was locked in a very small, very dark closet.

She began to wonder if her brother had changed his mind and headed for the surface—then she saw a dull glow outside the canopy. The light grew brighter until she could make out the flashlight’s beam as it drew closer. Though it seemed he had been gone for hours, it had been only a few minutes.

When Dirk climbed through the hatch a second later, she saw he was wielding a five-foot steel pole with a brass ball affixed to one end: the severed flagpole from the shipwreck. The submersible had struck bottom alongside the wreck, which Dirk had recognized through the bubble.

Dirk crawled forward and wedged the pole between the seat frame and the housing that clamped against Summer’s foot. Grabbing the opposite end, he pulled like an Olympic oarsman. The metal supporting the seat buckled immediately, allowing Summer to free her foot. She gave Dirk a hug, then signaled “Ascend” with an upraised thumb.

Dirk shined the light at the open hatch and gave her a shove. They had spent a dangerous amount of time at a depth close to three hundred feet and knew not to linger.

Summer waited outside the submersible for him, and they linked arms and began their ascent together. They kicked in a slow, measured pace, using their rising exhaust bubbles as a speedometer. Ascending too quickly would be a surefire recipe for the bends, and Dirk made sure they lagged behind the fast-rising bubbles.

It seemed to take forever. Summer was glad for the exertion, which slightly warmed her frozen bones, but her mind still wanted to play tricks on her, telling her they weren’t actually ascending or that they were actually falling back to the depths. It was the cold, she told herself, making her senses numb along with her extremities. She clung to Dirk, who moved like a robot, seemingly immune to the cold and dark.

At a depth of one hundred and fifty feet, the waters noticeably lightened as the surface light began to penetrate the deep. At one-twenty, they passed through a thermocline, and the water temperature warmed. And at eighty feet, Dirk ran out of air.

He wasn’t surprised. Because of his exerted swim to the wreck and back, he knew his air would fail before Summer’s. Drawing his hand across his throat to signal Summer, he jettisoned his tank and regulator. She passed her regulator over, and they began alternating breaths, unconsciously kicking more quickly toward the surface.