An evil-looking thing, McCarter thought, with all the tools of a predator: stereoscopic vision, a sleek, strong body, claws that resembled angled steel blades. Its mouth hung open on a set of powerfully muscled jaws and was filled with teeth like sharpened railroad spikes.

McCarter looked up toward the ceiling, upon which the animals had been crawling. The image of that ancient Mayan painting came to him, humans standing erect on the ground, blithely unaware of the Xibalbans directly beneath them, walking inverted with their feet on the underside of the earth’s surface. If I knew something like this lived down here, he thought, I’d consider this the underworld too.

As he shook off a chill, Danielle and Verhoven came up beside him, gawking at the thing, just as he had. Danielle seemed especially interested in the entry wounds from Verhoven’s shot. The damage resembled a pane of glass punctured by an errant baseball, with long fissures radiating from the wounds.

With the barrel of her rifle, Danielle reached out and pushed on the body. It was stiff. She tapped on it. It almost sounded hollow. “An exoskeleton,” she said. “Bones on the outside. Large animals don’t grow like this. Only insects and crustaceans.”

“It’s a damn gogga, then,” Verhoven said, using the Afrikaner slang for crawling bugs.

McCarter nudged Verhoven and pointed out a broad purple smear on his jacket, where the animal had hit him. The fibers of the coat were fraying and discolored, as if the smear was corrosive.

“Some kind of secretion,” Danielle said. “It’s all over the body of this one.”

As Verhoven pulled off the field jacket and tossed it aside, Danielle leaned closer to the animal. “Do you smell that?”

McCarter nodded. “Almost like ammonia,” he said. “I smelled it when the other one attacked us last night. But this is a lot stronger, even though this one is smaller.”

Danielle nodded, looking back toward the pools by the dam. “I was thinking the same thing,” she said. “And I think I know why. Ammonia is a base, an acid neutralizer. I think that’s what this thing is secreting—only, from the way it’s destroying that fabric, I’m guessing it’s a lot stronger than ammonia.”

“What good would that do them?” Verhoven asked.

Danielle nodded toward the pools. “That’s how they survive. They secrete this stuff to counteract the acid.”

McCarter remembered trying to help his son learn chemistry and repeated trips to the science department to ask fellow professors questions he could not answer. Bases were just as dangerous as acids. When the two were combined they would neutralize each other, but individually acids were corrosive and bases were caustic. Both were horrendously destructive to organic tissue and to materials far stronger than human skin. He turned to Danielle. Her calf was exposed where she’d cut off the torn section of the pant leg. Her skin was red but not blistering. “What about your leg?”

Danielle looked down; she guessed the torn section of her pant leg was fraying like Verhoven’s jacket, although she’d discarded it into the darkness somewhere. “I put peroxide on it,” she said. “I was thinking about infection, using it as an antiseptic, but peroxide is an acid, to some extent, it must have counteracted any of the base that got on my skin. It does feel strange though, almost like it were burning with a cold fire, like peppermint on the tongue.”

“Might want to use some more peroxide,” McCarter said.

As Danielle pulled out the plastic bottle, Verhoven held his hand out over the dead creature. “Notice anything else?” he said, looking at Danielle.

She shook her head.

“Dead animals radiate heat,” he said. “When you take one down, you can feel it pouring from the wounds. But not this thing.”

“What does that mean?” she asked.

“Cold-blooded maybe, or colder-blooded than we’re used to.”

“Might explain why the heat sensors have trouble picking them up,” Danielle said.

Verhoven pointed to the tail, where the tip split into a pair of spikes, like dual stingers. “Remind you of anything?”

Danielle nodded and McCarter thought of the body in the water with the two great holes in its chest, wounds from something that went in and came back out. It was terrible, a killing machine—but part of McCarter could not help but be in awe. “What the hell are these things?” he asked.

He exchanged glances with Danielle and Verhoven, but it was clear that neither of them had a clue.

A moment later Hawker joined them. He took a brief look at the animal. “Nice,” he said, sarcastically. “This trip is so much fun. Remind me to bring the whole family next time.” He turned to McCarter. “Let’s not forget why we’re down here.”

Despite an unshakable sense of awe they moved on, following the path that led beyond the plaza as it took them back into the deeper part of the cave. Soon, the craggy walls narrowed, closing in on them before becoming smooth with tool marks once again. They continued in a narrow valley that soon became a tunnel as the ceiling sloped down on them. The carved tunnel led to an even narrower rectangular doorway less than four feet high and perhaps eighteen inches wide at the most. They had to squeeze and duck to force themselves through. As soon as they reached the other side, a weak, raspy voice called out to them.

“Mr. Kaufman?”

McCarter responded. “It’s us, Susan. Not Kaufman.”

She stepped from the shadows. “Professor McCarter?”

“Are you all right?” he said.

She ran to them. Right into McCarter’s arms, who grabbed her in a bear hug, only slightly embarrassed. He could hear her wheezing and pulled out her inhaler, which he’d found and remembered to bring with him.

She used it immediately. “I heard the guns,” she said, her eyes welling up with tears. “I didn’t know if—”

She broke off her sentence, scanning the faces and stopping on Hawker’s. She seemed confused. “What are you doing here? What happened to Kaufman’s men?”

“Most of them are gone.” McCarter said. “Kaufman’s up on the surface. Brazos is guarding him. We heard you on the radio,” he added. “Apparently you couldn’t hear us.”

“I didn’t get any response,” she said. “Not sure I was using it right, and I think I killed the battery trying to call out.”

She went on to explain the attack, and details of her survival. “When it killed the other man, the radio came sliding across the floor and hit me. I grabbed it and I just ran,” she said. “I came down here and I found this door. It turned out to be a dead end in there, but by the time I tried to get out those things were trying to get in. They scratched and dug at the entrance for hours, but I guess they couldn’t fit through. So I stayed put.”

“That’s somewhat comforting,” Hawker said. “But we still have to go back that way to get out. And the sooner we go the better.”

Susan took McCarter by the hand. “Yes,” she said, very seriously. “But there are some things you have to see first.”

She led them deeper into the chamber, down a long, narrow passageway, past one empty room after another, rooms that had been cut from the rock itself, rooms with smooth, vertical walls and flat, even floors. It was a level of workmanship more advanced than that of the plaza outside. In fact, where Susan’s footprints had cleared the dust, the ground shone like expensive marble. McCarter bent to examine it, but Susan beckoned him to follow.

She pointed out a wall, covered with strange geometric symbols and, beside them, carved Mayan glyphs. And then she led them to a pile of debris where part of one wall and the ceiling had collapsed. She knelt down beside it.

McCarter paused, stunned. A figure lay there, half-buried in the rubble, partially hidden by the piles of rock. In the gray darkness it appeared to be the body of a child, but as the lights converged on the remains it became clear that it was something else.