“How sure are you about that, Doc?”

“The science is complicated, but believe me when I say this: as a biological weapon, LUCA is virtually useless on the large scale—especially when a thimbleful of anthrax could wipe out a city. But as an ecological threat, a weaponized version of LUCA is a thermonuclear bomb.”

“Then let’s make sure that never happens.”

“In regards to that, I’ve made some progress.”

10:48 P.M.

When Tucker and Bukolov rejoined the others, Anya was awake. Christopher guarded her with his AR-15 rifle, while Kane kept close watch.

Tucker ignored her and followed Bukolov to his makeshift office set up amid their stack of supplies. From the haphazard scatter of paper, notes, and journal pages, he had been busy.

“It’s here,” Bukolov said and grabbed De Klerk’s old diary from atop one of the boxes.

With the skill of a magician cutting a deck of cards, the doctor opened to the spot where it looked like pages had been cut out. He compared it to the pages Tucker had discovered.

“Looks like a perfect match,” Bukolov said.

Anya stirred, trying to see, to stand. But a deep-throated growl from Kane dropped her back to her butt.

“See. Here’s a crude, early rendition of LUCA in the old diary, a hazy sketch. A first-draft effort. What we had to work from before.” Bukolov fitted a sheet from Tucker’s collection into place. “This page was the diary’s next page. Before it was cut out. The finished masterpiece.”

The page in question depicted a deftly drawn sketch of a mushroomlike stalk with ruffled edges sprouting from a bulb. Colors of each structure were called out in tiny, precise print. Other drawings showed the same plant in various stages of growth.

Bukolov pointed to the earliest of the drawings. “This is LUCA in a dormant stage. A bulblike structure. De Klerk describes it here as a butter-yellow color. His measurements indicate it’s about the size of a golf ball. But don’t let its simplicity fool you. This structure is pure potential. Each cell in the bulb is a blank slate, a vicious chimera, waiting to unleash its fury on the modern world. It reproduces by infection and replication, as invasive as they come, an apex predator of the flora world. But if we could tame it, unlock the keys to its unique primordial genetics, anything could be possible.”

“But first we need to find it,” Tucker said.

Bukolov turned to him, a confused expression on his face. “I already explained where to find it.”

“When?”

“Just a moment ago, when I said, It’s here.”

Tucker had thought the doctor was referring to De Klerk’s diary. “What do you mean, it’s here?”

“Or it should be.” Bukolov stared around the cavern with frustration. “It is supposed to be here. In this cavern. At least according to De Klerk.”

“Why do you think that?”

Bukolov flipped the diary to the page before with the crude drawing of LUCA. “Here he talks about finding the dormant bulbs, but he never says where to find them. He’s a sly one. But see here in the margin of that section.”

Tucker leaned over. He couldn’t read the passage written in Afrikaans, but next to it was a crudely scribbled spiral.

“I always thought it was just an idle doodle,” Bukolov said. “I do it all the time. Especially when I’m concentrating. My mind wanders, then so does my pen.”

“But you think it’s significant now.”

“The drawing looks like water spilling down a bathtub drain.” Bukolov pointed to the torrent of water across the room. “It wasn’t a mindless squiggle. De Klerk was symbolically marking this passage about the discovery of the bulb with its location. As I said, it’s here. Under the bathtub drain.”

Bukolov closed the journal and tossed it aside. “I just have to find it. And now that I don’t have to play babysitter . . .”

With a glare toward Anya, Bukolov picked up an LED lantern and set off across the cavern.

For the moment, Tucker left the doctor to his search. Knowing now that Kharzin’s team was in the neighborhood, he had to prepare for the contingency that Bukolov might fail. His ruse with the booby-trapped Rover would not stop the enemy for long . . . nor did he know how many of the enemy his trick might take out.

He pictured his last glimpse of Felice Nilsson, leaning out the helicopter door, her lower face hidden by a scarf, her blond hair whipping in the wind.

It was too much to hope that she would be caught in that blast.

He had to be ready.

He crossed to the pile of boxes and packs, knelt down, and pulled over the stiff cardboard box holding the blocks of C-4.

“Christopher, can you start measuring out six-foot lengths of detonation cord? I’ll need about fifteen of them.”

Anya stared at them, her face unreadable.

Ignoring her, he calculated the best spots to set his charges to cause the most destruction. If Bukolov couldn’t find the bulbs of LUCA, he intended to make sure no one ever did, especially General Kharzin.

He unfolded the flaps of the box of C-4 and stared inside.

With a sinking drop of his stomach, he glanced again over to Anya. Her expression had changed only very slightly, the tiniest ghost of a smile.

“How?” he asked.

The box before him was packed full of dirt, about the same weight as C-4.

Anya shrugged. “Back at the campsite this morning, after you left. All your C-4 is buried out there.”

Of course, she had known of his contingency plan to blow the cave as a fail-safe and had taken steps to ensure it wouldn’t happen.

But she was wrong about one fact: all your C-4.

He had taken a block of the explosive with him as he hunted for guerrillas and ran into the pack of dogs. Later, he used half of it to rig the Rover. He still had the other half, but it was far too little to do any real damage here.

And now they were running out of time.

If they couldn’t blow the place up, that left only one path open to them: find the source of LUCA before Kharzin’s team returned.

So there was still hope—not great, but something.

Bukolov dashed it a moment later as he returned with more bad news. “I found nothing.”

11:12 P.M.

Fueled by anger and frustration, Tucker tossed the leg of an old broken chair across the cavern floor. It bounced and skittered away, splashing through a standing pool of water. Christopher and Bukolov worked elsewhere in the cavern, spread out, slowly circling the torrent of water falling through the room’s center.

Tucker wasn’t satisfied with Bukolov’s cursory search.

He had them sifting through some of the old Boer detritus and flotsam tossed against the walls by prior flooding.

But it was eating up time and getting them nowhere.

If they had a sample of LUCA, Kane could have quickly sniffed out the dark garden hidden here, but they didn’t. So he left the shepherd guarding Anya.

Christopher and Bukolov finally reached him. They’d made a complete loop of the cavern. He read the lack of success in their defeated expressions.

“Maybe I was wrong about the bathtub drain.” Bukolov stared up at the water cascading through the vortex. “Maybe it was just a doodle.”

Tucker suddenly stiffened next to him. “We’ve been so stupid . . .”

Christopher turned. “What?”

Tucker grabbed Bukolov by the shoulder. “De Klerk was marking the location. It is a drain.”

The doctor looked up again toward the ceiling.

“No.” Tucker pointed to the floor, to where the flood of water flowing down from above either pooled—or drained through fissures in the floor. “That’s the drain depicted by De Klerk. The water must be going somewhere.”

Bukolov’s eyes went wide. “There’s more cavern below us!”