What she saw still boggled her mind. There was a herd of enormous, thick-bodied creatures, as big as elephants, but with twenty-foot long necks and tails that were even longer, calmly grazing on the strange vegetation sprouting from the cavern floor. They were unbothered by the flame jets. Creatures with ridged backs that reminded her of stegosauruses, though she knew they were something different, roamed across the landscape. Every few seconds, dark shapes leapt from the ground in a flutter of outstretched wings, gliding up toward the high ceiling, and dropping back down. The raptors seemed to be everywhere, darting their heads at the ground, as if pecking for insects and worms, and mostly leaving the larger dinosaurs alone. But given their earlier ferocity, she had no trouble imagining a pack of them taking down a juvenile. And out at the limit of her vision, something very large moved, swift and low to the ground, like a lion stalking its prey.
“Okay,” she said. “If the impossible is possible, the question is ‘how?’”
“Is it a question that you have to answer right now?” Bishop said, pulling her back into the shadows.
She hushed him, not for fear of alerting the raptors, but so she could think.
“Habitat and food. They have both down here. Maybe this cave was here sixty-five million years ago. Maybe that’s how they survived.” She shook her head. It was an oversimplified explanation. The Cretaceous-Paleogene extinction had been a lot more than just a huge explosion. The dust cloud from the asteroid impact had shut out the sun for nearly a decade, interrupting photosynthesis and demolishing the foundation of the food web.
“The food web!” She turned to Bishop, eager to share her revelation. “Don’t you see? This place is a self-contained ecosystem. The dinosaurs that survived adapted to conditions here. That’s why they never migrated away.”
She stopped, realizing that even that was a little too simplistic. “An ecosystem begins with producers — plants. But plants need sunlight to grow… unless…” She stepped back out onto the ledge and peered down at the weird vegetation growing in the vast flame-lit plain. “Where did those fires come from?”
“I don’t know. Maybe the dinosaurs are rubbing sticks together.” Bishop pulled her back again.
“Blue and yellow flames,” she continued, still thinking aloud. “Pure ethane burns blue, and methane burns yellow. Those are gas fires, natural gas fires. And they’ve probably been burning for…” She turned to face him as yet another realization dawned.
“Oh, my God. That’s it.” She removed the backpack she’d brought from the expedition camp and unzipped it for the first time since fleeing. As she opened her MacBook Pro laptop computer and booted it up, she could almost feel the irritated stares of the others, but there was no way to explain it simply. The screen lit up, glowing just slightly brighter than Knight’s bundle of chemlights. When the boot sequence was complete, she opened the file containing all the data the expedition had gathered.
She turned the screen so they could follow along. It showed a picture of what looked like pink donut sprinkles on a white background. “This is the bacteria we recovered from the bottom of Lake Kivu. It’s a variant of Escherichia coli that has adapted to the extreme conditions at the bottom of Lake Kivu…” She could see that she was already losing them. “All life forms need energy to live. Normal E. coli, like the kind we have in our intestines, relies on our body temperature to stay alive. The organisms at the bottom of Lake Kivu get their energy from the chemical reactions of volcanic gases filtering up through the lake bottom. They produce their own food supply through the process of fermentation.”
“Fermentation. You’re saying they produce alcohol?”
She smiled, pleased that Bishop understood. He was much more than a grunt, as was Knight. “Exactly. All cellular organisms ingest carbon compounds and convert them into a fuel called ATP, either through respiration or fermentation. Fermentation isn’t as efficient as respiration, but when there’s an abundant food supply, that doesn’t matter as much. A by-product of fermentation is hydrocarbons, like ethanol — alcohol — or sometimes methanol compounds. That’s why decaying organic matter produces methane. It’s what’s been happening at the bottom of Lake Kivu, only the microbes aren’t subsisting on organic carbon. They’re getting it from volcanic outgassing.”
“Okay. What’s that got to do with dinosaurs?”
“For dinosaurs to live down here, to actually thrive down here, requires a complex eco-system.”
“I get that,” he replied. “They have a food chain. Plant-eaters and meat-eaters.”
“It’s more complicated than that. In an ecosystem, energy is lost as it moves from one trophic level to the next, roughly speaking by a factor of ten. It’s like a pyramid where each level is only a tenth as big as the one below it. One meat-eater needs ten times its mass in plant-eaters, and each plant-eater needs ten times its mass in plants. And the amount of available energy in those plants — in terms of calories — is about a tenth of what the plant needs just to survive. On the surface, the plants get that energy from sunlight. If any part of the pyramid is disrupted, the whole system collapses.
“The dinosaurs went extinct because of widespread climate change, probably the result of the Chicxulub asteroid impact, which prevented plants from growing. The food web collapsed.”
Bishop nodded slowly. “So for there to be that many raptors down here, there have to be even more prey animals for them to eat.”
“Right, but the plants are the important thing. They’re the base of the food pyramid.” She crawled back to the edge of the recess and pointed out at the landscape below. “Ordinary plants require sunlight for photosynthesis. Whatever those things are, they’re getting their energy from some other source.”
“The light from the fires?”
“Maybe. They might not be photosynthesizing plants at all, or if they are, they’ve evolved over the last sixty-five million years to be able to use that energy more efficiently. The animals would have had to undergo adaptive changes as well. That’s why they never migrated back to the surface. They might wander out once in a while, but this is their primary habitat. They’ve got everything they need down here.”
She watched the raptors roaming the plain. Those closest to the cavern wall were still motionless, alert to the presence of intruders, but most of the others were busy dipping their heads and scratching at the vegetation, searching for prey. “But it’s the base of the pyramid that matters most. For those fires to be burning like that… the amount of microbial metabolic activity must be off the scale. That’s what we were looking for at Lake Kivu: a bacterial organism that could produce hydrocarbons on a commercially viable scale.”
“What do you mean by that?” Bishop asked.
“We already know how to use microbes to make fuel. They do it naturally. The problem with biofuel production is the same as with the food web. You have to put more energy into a system than you get back from it, not to mention that the land used for growing your fuel crop isn’t available for food production.”
“You think the bacteria in this cave have figured out a way to make fuel more efficiently? Like a closed system?”
“I do. And I’d bet money that the microbes in the soil of this cave are identical to the extremophiles we found in Lake Kivu.”
Bishop pondered the idea for a moment, then shook his head. “As much as I hate to stand in the way of scientific progress, right now our only priority is getting out of here.”
Felice started to protest, but then realized that she had gotten carried away by the euphoria of discovery. She closed the computer and slid it back into her pack.