And then there are a couple of paintings that can only be described as masterful. They’re done on rough material, inexpertly mounted, and attached to the rock walls with adhesive . . . but the skill is astounding.

Each picture shows an animal in the center, in a pose that suggests it has no idea anyone is watching. A leopard lolling in indolent magnificence; a squirrel hanging by its hind feet, nibbling a nut held in its forepaws; a dolphin breaching the surface of the foamy sea just enough to snatch a breath, its eye murky beneath the water.

Near the beasts, their environment is depicted in vivid, minute detail. The leopard’s jungle is lush and green; the dolphin’s sea is speckled with sargassum and silver fingerlings. But as the color stretches to the outer edge of the canvas, the details begin to fade. The richness is diminished, the colors become muted. Their world is vanishing. Suddenly I can see prescience in the animals’ eyes. They know their existence is ending.

In the right-hand corner, in scratchy black letters, is the artist’s name. Lachlan.

“These are incredible,” I say, and I feel like that pat praise sounds insincere. I want to gush about them, about what his paintings make me feel, the autumnal nostalgia for something I never experienced, the loss that happened before my time. But I feel shy, and can’t find the words.

Lachlan shrugs. “Just something I do in my spare time,” he says, dismissing his own skill. “Not that I have much of that.”

I try to explain my reaction to the paintings. “They capture that feeling of things slipping away, the inevitability of it . . . There was a time, I’m sure, when people knew the end was coming, that there was no stopping the Ecofail. When they still had their cars and air-conditioners and pesticide-laden crops and could pretend everything would go on like that forever, but they knew the edges of their world were dissolving, and there was nothing they could do about it anymore.” I’m frowning, struggling to express myself.

“One man could have done something about it,” Lachlan says staunchly. “Aaron Al-Baz tried to stop the man-made catastrophe, and no one would listen. He couldn’t stop them from destroying the world, but he could save them. Save us. We have to live up to his memory so we’re worthy when we can finally go out into the world, when it heals.”

“He’s your hero, isn’t he?” I ask softly.

“He’s everybody’s hero.”

“What if he wasn’t?” I force myself to ask.

Lachlan is fiddling with his weapon, doing something the purpose of which I can’t fathom, but it is taking his concentration. He seems to think I’m suggesting something hypothetical, an intellectual exercise in debate.

“If he wasn’t a hero, why were our ancestors all saved in the final human refuge: Eden?” he asks absently as he makes a minute adjustment to the sight of his weapon.

I make myself speak more firmly. “Lachlan, I mean it. What would happen to the people of Eden, of the Underground—to you—if you found out that Aaron Al-Baz wasn’t the good man everyone thinks he is?”

I have his attention now. His head comes up, his entire body is tense like he just spotted danger.

“The fact that we’re here, when every other animal on the planet is gone, seems to me like proof of his goodness.” His voice is challenging, almost antagonistic, and a part of me wants to let the whole thing go. “If the people of Eden found out otherwise . . .” He looks confused for a second. He can’t conceive of quite what I’m getting at, or what the consequences might be. “It would disrupt everything. Everything that everyone believes in.”

“I found something,” I say in a small voice. “My mother gave it to me, before she . . .” I swallow hard. “She found it in our house, hidden behind a stone wall. The house belonged to Aaron Al-Baz just after the Ecofail.”

And so I sit down on his bed and tell him what I learned.

Aaron Al-Baz was a visionary, who saw the imminent destruction of the global ecosystem long before anyone else. He was a genius, who put technology to work, first to stop the devastation and then, when that proved impossible, to fix it. He made the machines, the computers, the programs, work for him to begin the generations-long process of saving the planet. But before that, he decided to get rid of the thing that had poisoned and burned the planet in the first place.

It wasn’t the Ecofail that killed off the human species. It was Aaron Al-Baz. He did it to save the rest of the planet.

It made sense, in a sick, inhumanely logical kind of way. The planet was dying because of people. He could either try to fix the harm humans had done . . . or go straight to its source. At some point the brilliant, mad genius developed a virus that would kill nearly 100 percent of the population. His talent knew no bounds; he simply created a program that would devise an unstoppable pathogen, and let his mechanical minions create it. Then when the scientists unleashed their particles into the atmosphere in an attempt to reverse global warming, Al-Baz released his disease.

Of course, he made sure that he himself was immune to it. His family, too. As for the rest of humanity, he left that up to chance.

Natural selection, he calls it in his manifesto.

He reduced the human population to a fraction of its original billions. Then he gathered up some of the survivors and installed them in Eden, to await the day humans could repopulate the world.

And the rest of the humans who survived the plague? The ones he didn’t take into Eden? They were left to fend for themselves, to die slowly in the dying world along with all of the other animals.

“No,” Lachlan says flatly when I finish telling him what I’ve learned. It is the first word he’s spoken throughout the tale. The whole time he sat still and silent on his bed next to me, almost expressionless except for a slight downward turn of his brow.

“But I have proof,” I insist, thinking he doesn’t believe me. “Aaron Al-Baz’s own admission. I can show you.” I start to get up, but he pulls me back down onto the bed.

“No,” he says again, softly but so firmly.

“But . . .”

He holds my hand in his, so I don’t even try to rise again. I look down at his knuckles, at the seams of white scars where the skin has been split countless times. So many punches, so much fighting. That has been his life. But he’s calm now. Almost unnaturally calm. The only sign of his agitation is the nervous way he strokes my skin with his thumb, over and over again in the same place.