“How the hell could you do that to me?” I ask, my voice furious and hurt. “Lachlan, I thought this was supposed to be a brotherhood, a family of second children. I trusted you with everything about myself. Why couldn’t you trust me?”

I expect an apology, but he looks at me levelly and says, “The Underground is bigger than any one person, more important than one night of suffering. We’ve only known about you for a matter of days, so we know less about you than we typically do about second children. Most we find when they’re very young, even babies, or before birth. Our hunting methods are sophisticated—more so than the Center’s, anyway. But we missed you entirely, and only found you by luck. Most second children come here so early they are part of the family. They’re loyal. But you . . .”

“We don’t know where your loyalties lie,” Flint supplies.

“I’ve been a prisoner all of my life because of Center policies!” I rage. “I’ve been in constant danger of prison or death. The government killed my mother! Do you really have any doubt about my loyalties at this point?”

“People can be surprising—even to themselves,” Flint says. “You don’t know what you’d do in the worst situations, until they happen to you. But for now, I’m willing to let you into the Underground. You’re one of us . . . sister.”

He offers me his hand. I stare at it, considering. I understand why he did what he did. I really do. In theory. But the fact that he did it to me makes it different. There’s a world of difference between what is rationally necessary and what a good person should actually do. Logic shouldn’t always win.

But my hand rises and clasps his firmly. Something seems to surge through me. Flint is a natural leader, I can tell. Just looking at him makes me feel like he has everything firmly under control. He’s inspiring, and I feel like I can rely on him. Sister . . . I’m no longer alone.

But when Lachlan holds out his hand, too, I glare coldly at him and then deliberately look away. We confided in each other. We talked about our lives. He shouldn’t have let that happen to me. I understand why Flint did it, and forgive him, but somehow I can’t forgive Lachlan. It might not make perfect sense, but there it is.

“Come,” Flint says, touching me lightly on the shoulder. “Let me introduce you to the Underground.” We exit the torture cave—just a room now—and step out into something I could never have imagined.

I’m inside a jewel. A faceted, glowing, many-hued jewel.

“Are . . . are we still under Eden?” I stammer, not believing my eyes.

I look out over a huge crystal cavern, maybe half a mile across. The entire roof and most of the walls of the gigantic cave are covered with clear bright jutting stones that look like colored ice. In subtle shining shades of palest pink and amethyst, of smoky silver and water-blue and pure clear diamond they surround me, catching the dim artificial light, so beautiful that for just a second I don’t notice the even more remarkable thing below them. In the center of the twinkling crystal cavern, rising almost to the ceiling and spreading its canopy more than a hundred feet across, is a tree.

A tree. A living tree.

The trunk is massive, twenty, thirty feet across, lumpy and gnarled. Roots spread aboveground for around the trunk before plunging into Earth. Earth? Dirt? It can’t be. The ground of the cavern looks like a forest floor, Earth covered in brown fallen leaves.

My eyes rise to the tree again, and for the first time in my life I make the gesture they make in temple. My fist rises from my belly to my face, my fingers branch out, like a seed growing, sprouting. I feel reverent awe, like I should fall to my knees, hide my face in the presence of something so radiantly beautiful, so perfect as a tree.

Then dawn breaks over the green, living canopy, making the crystals above seem to dance, and tears fall silently from my eyes.

“It can’t be real,” I whisper. But I can smell a sharp-sweet scent, and beneath that something rich and moist. Leaves, and Earth. I’ve never smelled anything like that—no one has, for generations—but some part deep within me recognizes the scents right away. Somewhere in my blood is a memory of nature, and it rejoices.

The sky goes from gray to pearl touched with pink as the sun breaks an unseen horizon and bathes the world with gentle morning light. That part has to be illusion, technology. We’re deep underground, with rock all around us. Somehow they’ve made an almost perfect simulacrum of breaking dawn. But it’s not just color, or light. I feel a flush of warmth hit my skin from where the sun is rising. The crystals in the roof and walls shine brightly beautiful.

“The tree is real,” Lachlan says from beside me. I’m so awestruck I don’t even think to move away from him. “And the Earth.”

“But . . . there aren’t any more trees.” That’s what we’ve been taught. The world is dead, the dirt is toxic, all living things extinct except for a few hardy lichens, single-celled organisms . . . and a handful of humans.

“There’s one,” Lachlan says.

“But how?”

“Aaron Al-Baz, of course,” Flint says, his voice low and reverent. “The man who saved us. The man who will save the world. He made a perfect Eden, and humans corrupted it. We mean to bring Eden back to the paradise he intended it to be.”

“What is this place?”

“The backup Eden,” Flint answers. “This is where humans would have had to live if Eden wasn’t ready in time, or if the world was more toxic than predicted. Underground. He kept it as a secret fail-safe in case humans managed to ruin things once again on the surface. It is self-contained and self-perpetuating, set up on computer controls and automation entirely separate from EcoPan.”

That’s amazing, I think. We’ve been taught that EcoPan took over control of every computer and electronic system on the planet.

“But he knew man can’t live completely apart from nature,” Flint went on, “so he managed to preserve this tree. The dirt is real organic soil from the surface, clean, good, uncontaminated pre-fail dirt. It goes down fifty feet, so the roots can bury deep. Hidden panels among the crystals simulate sunlight. As far as the tree knows, it’s still on the surface. It gets sunlight, water, nutrients, seasons . . . and it gives us almost all the oxygen we need to survive down here with the place entirely sealed.”