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.