Sam and Remi swam cautiously in the deepening darkness, just going with the current for about a hundred feet to make sure that no one standing above the cenote could see them turn on their flashlights, and they increased their speed to move along the stone corridor of the underground river. The water rose all the way to the ceiling of the cavern, leaving no airspace above the surface. At first, the walls were about twenty feet apart but thirty or forty feet deep. Each time the space between the walls narrowed, Sam and Remi would feel a growing dread. When the space opened up a little, their relief was intense.
They kicked their fins steadily to keep their speed up, and the current helped them along. They held their flashlights ahead of them, but what they saw was always the same — more curving tunnel. When the tunnel narrowed, Sam would wonder whether it was merely a fissure in the rock opened up by one of the frequent earthquakes in the region. If it was, it could narrow at some point from twenty feet down to six inches, and they would be trapped and drown.
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.”
They tried the experiment, but there was no light. They waited for their eyes to adjust to the darkness, but they still detected nothing. They switched on their flashlights again. “At least we can swim on the surface for a bit,” said Sam. They closed the valves on their tanks and began to move.
The space remained above them, and they breathed the air and swam steadily along with the current.
Sam paused. “I think I know what this is.”
“You do?” she said.
“Rainwater that flows into cenotes or seeps in through cracks feeds the river. The water level must be very high after a rain — maybe even through the rainy season — and then gets lower as time passes.”
“Sounds right,” she said. “It would explain why the Mayans built those big stone strips like gutters, to catch the rain and direct it to the pool.”
“When there’s no rain for a while, much of the underground river probably gets low, and air flows in above it. When the river rises again, air gets trapped in places like this,” Sam said. “We’ve got to try to stay on the surface as long as we can to save our air.”
“And by the way,” said Remi, “don’t make any more moves like trying to give me your air tank. I’m already aware that chivalry isn’t dead.”
“I was just being rational,” he said. “You use less air than I do, so you could make it last longer and get farther.”
“All that would accomplish is that we’d both die alone. I plan to die in front of an audience and I picked mine years ago. You’re it.”
“Saves you having to send out a lot of invitations,” he said.
“That’s right,” she said. “This is hard enough with you alive. Just stick with me and curb your generosity.”
They swam on along the curving tunnel for an hour until they came to a spot where the wall ahead reached all the way down under the water. They stopped and held on to the wall long enough for an awkward kiss. Then they lowered their masks over their faces and turned on the valves of their tanks. Remi said, “Remember, it’s both of us or neither,” and put her mouthpiece in.
They sank, and found themselves in a long passage that looked exactly like the stretches they had first passed through. As they swam, Remi wished she had looked at her watch before they had submerged. She had timed their arrival at the air pocket at sixteen minutes, but how much time had gone by? And did their tanks actually hold nine more minutes of air? She and Sam had never tested the limits before. Letting their air get this low would have been risky and stupid on any day when they could have simply surfaced and gotten fresh tanks from the dive boat.
There was nothing she could do but swim. As the minutes ticked away, the passage opened into another, wider space. The bottom of the river was oddly uneven, with loose chunks of rock instead of the smooth-worn riverbed they’d seen before. Then she realized she was seeing these things outside the perimeter of their flashlight beams — real light was filtering down from above. They swam upward. As the light grew brighter, Remi laughed and heard herself make a squeaky noise like a dolphin. She saw Sam huff out a big flurry of bubbles in an answering laugh, and they broke the surface smiling.
But Remi’s laugh caught in her throat. There was light in this dome, directly above their heads, coming from a circular hole that opened to the starry sky. But the hole was at the center of the dome, beyond their reach, at least six feet above the surface of the river.
“Now, there’s a problem,” Sam said.
“What can we do?”
“I’m going down to take a look around. Stay here for a minute.” He lowered his mask again and submerged. Remi waited until he surfaced again.
“Well?” she said.
Sam swam over to the side of the stone riverbed. It seemed to rise in the water, then rose partway out of it, so he was only up to his waist. “I’m standing on a pile of rock. At some point, a pretty big chunk of wall came down right here. There’s also a pile in the center, right below where the roof collapsed.”
“Very dramatic,” she said. “Does this mean we’re not going to the great beyond?”
Sam looked up at the hole in the dome. “I think it does, but we’ll have to work pretty hard to get out. Get ready to move some stones.”
They dove to the bottom, where Sam had been standing, and began to move chunks of stone from the pile along the wall to the spot just below the opening. Sam moved the largest chunks he could, rolling them end over end, to add to the pile in the center. Soon he took off his fins and worked in his booties. It was clear that at some point part of the wall had collapsed to make the pile, and stones gradually falling from the ceiling formed the cenote, with even more stones coming down as it enlarged. Sam and Remi both were free diving and they had to stop occasionally to catch their breaths.
When they had moved the whole pile of stone from the place where it had fallen to the place they wanted it,
