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, they stopped at the surface. “We’re running out of stones,” said Remi.

“I think we’ve got to bet the rest of our air on finding more and building higher.”

“I’m for risking it,” she said. “This is the only chance we’re likely to get.”