Sam kept checking his watch as they swam. He and Remi had made a dive yesterday morning that had lasted about fifteen minutes. Each of their aluminum tanks still held about twenty-five minutes of air. That meant that for the first twelve minutes, if they reached an obstacle, they might still be able to swim back to the cenote and surface. Maybe if they did, they’d find that the men who had been after them had already stormed the enclosure, seen they were gone, and left to search for them. Sam knew this thought was part fantasy and part nightmare: the possibility that betting their lives on this underground river might be a dead end.

And then it was thirteen minutes, and he knew that if they tried to swim back, they probably wouldn’t make it before their air was gone. After five more minutes, they certainly wouldn’t.

When twenty minutes had passed, they could count on only five more minutes of air. Even that might be optimistic. They had been swimming steadily, so they had used air at an accelerated rate. He thought about their chances as rationally as he could. There was no reason to believe that they would reach another opening in the ground above them in the next five minutes. Remi was smaller and lighter and used less air than he did. If she had both tanks, she would have twice as much time to find an escape.

Sam shifted his tank to the side so he could turn off the valve, but Remi saw what he was doing. She grasped his wrist with surprising strength and shook her head violently. Sam realized that she must have been thinking the same thoughts, feeling the same fears, and known that Sam would try to give her his tank.

When Remi had grasped Sam’s wrist, his flashlight had swept the space above them, and something had looked different. Now he looked back and upward. He had gotten used to the sight of the bubbles they exhaled rising to the ceiling of the cavern, sliding into a depression, and staying there as a single, gelatinous bubble. Now his bubbles disappeared. He swam upward, with Remi still holding his wrist.

They broke the surface together and aimed their flashlights upward. They were in a dome, with the limestone ceiling about ten feet above their heads. Sam removed his mouthpiece and cautiously took a shallow breath. “The air is good,” he announced.

Remi took out her mouthpiece. They raised their masks and looked around. “I was afraid it would be carbon monoxide or hydrogen sulfide or something from a volcano,” she said.

“Nope. Just air.”

“It’s sweet, clean air,” she said. “How is it getting in?”

“Let’s turn off the flashlights and see if light is coming in.”