“Sybil, why won’t the door open?”

Only essential personnel are allowed to circulate right now, Konstance.

She sees Ezekiel wince at the lights, fall off his stool. Is it safe for Father to be out there? Is it safe in here?

She steps onto her own Perambulator, beside her mother’s, and touches her Vizer.

In the Library grown-ups gesticulate around tables while cyclones of documents whirl above them. Mrs. Chen herds the teenagers up a ladder to a table on the second tier and sets an orange volume in the center. Ramón and Jessi Ko and Omicron Philips and Ezekiel’s little brother Tayvon watch as a foot-tall woman in a light-blue worksuit with the word ILIUM stitched on the breast emerges from the book. If at some point during your long voyage, she says, it becomes necessary to quarantine in your compartments, be sure to stick to your routines. Exercise daily, seek out fellow crew members in the Library, and…

Ramón says, “You hear about people vomiting but to actually see it?” and Jessi Ko says, “I hear Quarantine One lasts seven days no matter what,” and Omicron says, “I hear Quarantine Two lasts two months,” and Konstance says, “I hope your brother feels better soon, Tayvon,” and Tayvon bunches his eyebrows like he does when he’s concentrating on a mathematics problem.

Below them Mrs. Chen crosses the atrium and joins grown-ups around a table, images of cells and bacteria and viruses rotating in the space between them. Ramón says, “Let’s go play Ninefold Darkness,” and the four of them scamper up a ladder toward the Games Section, and Konstance watches the flying books a moment longer, then takes a slip of paper from the box in the center of the table, writes Atlas, and drops it into the slot.

“Thessaly,” she says, and drops through the Earth’s atmosphere and floats over the olive-and-rust-colored mountainscape of central Greece. Roadways emerge below, the terrain cut into polygons by fences, hedgerows, and walls, a familiar village coming into view now—cinderblock privacy walls, slate rooftops beneath cliff faces—and she’s walking the cracked pavement of a rural road in the Pindus Mountains.

Side streets split left and right, little dirt thoroughfares branching off those, drawing an elaborate tracery higher into the hills. She climbs past a row of houses built right up to the roadside, a disemboweled car in front of one, a face-blurred man in a plastic chair in front of the next. A houseplant wilts in a window; a sign with a skull on it has been mounted on a pole out front.

She turns right, following a route she knows well. Mrs. Flowers was right: the other kids find the Atlas hilariously obsolete. There’s no jumping or tunneling like in the more sophisticated games in the Games Section: all you do is walk. You can’t fly or build or fight or collaborate; you don’t feel mud grab your boots or raindrops prick your face; you can’t hear explosions or waterfalls; you can hardly leave the roads. And inside the Atlas everything besides the roads is as immaterial as air: walls, trees, people. The only solid thing is the ground.

Yet it fascinates Konstance; she cannot get enough of it. To drop feet-first into Taipei or the ruins of Bangladesh, a sand road on a little island off Cuba, to see the images of face-blurred people frozen here and there in their old-fashioned outfits, the pageants of traffic circles and piazzas and tent-cities, pigeons and raindrops and buses and soldiers in helmets frozen mid-gesture; the graffiti murals, the hulks of carbon-capture plants, the rusted army tanks, the water trucks—it’s all there, an entire planet on a server. Gardens are her favorite: mango trees on a median reaching toward the sun in Colombia; wisteria heaped on a café pergola in Serbia; ivy swarming up an orchard wall in Syracuse.

Just ahead an old woman in black stockings and a gray dress has been captured by the cameras halfway up a steep hill, her back hunched in the heat, wearing a white respirator mask and pushing a baby stroller full of what look like glass bottles. Konstance shuts her eyes as she walks through her.

A high fence, a low wall, and the road thins to a track switchbacking up through mixed vegetation. A silver sky plays overhead. Strange bulges and shadows lurk behind trees where the software pixelates, and as the trail climbs it continues to thin, the landscape growing more desolate and windswept, until she reaches a place where the Atlas cameras went no farther, and the trail peters out at a massive Bosnian pine, probably twenty-five meters high, twisting up toward the sky, like the great-great-grandfather of her sapling in Farm 4.

She stops, inhales: a dozen times she has visited this tree, seeking something. Through the gnarled old branches the cameras have caught a great cavalcade of clouds, and the tree clings to the mountainside as though it has grown there since the beginning of time.

She pants, sweating on her Perambulator inside Compartment 17, and leans as far forward as she can to touch its trunk, her fingertips passing through, the interface breaking down into a grainy smudge, a girl alone with a centuries-old pine tree in the sunbaked mountains of Thessaly, land of magic.

* * *

Before NoLight Father comes through the door of Compartment 17 wearing an oxygen hood with a clear visor and a cyclopic headlamp. “Just a precaution,” he says, his voice muffled. He sets three covered trays on Mother’s sewing table as the door seals behind him, sanitizes his hands, and removes the hood.

“Broccoli cacciatore. Sybil says we’re moving to printers in each compartment to decentralize meals, so this might be our last fresh produce for a bit.”

Mother gnaws her lips. Her face is as white as the walls. “How’s Ezekiel?”

Father shakes his head.

“It’s contagious?”

“No one knows yet. Dr. Cha is with him.”

“Why hasn’t Sybil solved it?”

I am working on it, says Sybil.

“Work faster,” says Mother.

Konstance and Father eat. Mother sits on her bunk, her food untouched. Again she asks Sybil to check their vital signs.

Pulse and respiration rates normal. Blood pressure shipshape.

Konstance climbs into her berth and Father stacks the trays by the door, then rests his chin on her mattress and pushes her curls out of her eyes.

“On Earth, when I was a boy, most everybody got sick. Rashes, funny little fevers. All the unmodified people got sick every now and then. It’s part of being human. We think of viruses as evil but in reality few are. Life usually seeks to cooperate, not fight.”

The diodes in the ceiling dim and Father presses a palm to her forehead and in a great dizzy uprush comes the sensation of standing inside the Atlas atop the Theodosian walls, all that white limestone crumbling under the sun. For as long as we have been a species, Mrs. Flowers said, we humans have tried to defeat death. None of us ever has.

* * *

The following morning Konstance stands in the Library at the second-tier railing with Jessi Ko and Omicron and Ramón waiting for Dr. Pori to arrive and commence the morning’s lesson in precalculus. Jessi says, “Tayvon’s late too,” and Omicron says, “I don’t see Mrs. Lee either, and she was the one with Zeke’s chunder all over her,” and the four children fall quiet.

Eventually Jessi Ko says she’s heard that if you feel sick you’re supposed to say, “Sybil, I’m not feeling well,” and if Sybil detects something wrong with you, she sends Dr. Cha and Engineer Goldberg to your compartment wearing full biohazard containment suits, and Sybil will unlock the door so they can isolate you in the Infirmary. Ramón says, “That sounds awful,” and Omicron whispers, “Look,” because down on the main floor Mrs. Chen is leading all six members of the crew who have not yet turned ten across the atrium.