The balloon was collapsing.

They worked grimly, dragging themselves into the rigging and cutting away the burning wicks. The envelope crumpled over the doused lamps.

And Boyd was upside down.

Or Allel was.

Her harness was slack. The components of their balloon drifted in a jumble. Boyd thrashed in the air as if drowning — but there was no up to kick towards. Fear showed beneath her pale scars.

But Allel understood.

“It’s the middle of the Gap!” Allel yelled, exhilarated by her mother’s discomfiture. “The Shell dwellers live upside down. Up for us is down for them. Did we think we’d fly up and bump against the Shell like a ceiling? This is the place where up and down cross over!” Warm air spilled from the balloon and brushed her face. Ground and Shell were enormous parallel plates that careened identically around her. She laughed and swooped.

But their equilibrium in the weightless zone was unstable, and soon invisible fingers clutched at them. Wind whistled in the tangled rigging and their harness grew taut again. “We’re falling back!” Allel cried in disappointment. Boyd struggled to keep her good arm free.

Now air resistance roughly righted them. The balloon opened out like a parachute but scarcely slowed their fall.

Boyd roared above the wind: “We’ve got to light the burners!”

They hunted for flints and cupped their hands around the wicks to keep out the snatching breeze. Heat roared up. Boyd thrust at the fuel pumps while Allel scrambled precariously into the tangled rigging to drag at the neck of the envelope, trying to trap all the warmed air.

Their descent slowed a little. Allel’s arms ached and her hair whipped at her forehead. The ground exploded into unwelcome details, rivers and hills and trees and pebbles—

She rolled on impossibly hard earth, grass blades clutching at her face. Her blood was loud in her ears. The balloon folded as if wounded.

In a sunlit meadow, mother and daughter lay amid the ruins of their bark spaceship.

Sunlight scoured her eyes. Allel sat up, blinking, pushing at the knotted remains of her harness. She was surrounded by cool grass and flowers; a brook led to a stand of cow-trees and the horizon was made up of heather-coated hills.

And, as it had always done, the Shell curved over it all like a great blue tent.

Boyd slept peacefully in a tatter of the balloon. Allel hesitated for some minutes, vaguely fearful of her mother’s reaction. Then she found a remnant of a shattered burner and woke her mother with a cup of brook water. Boyd sat up clumsily, favoring her bad arm.

“We failed,” Allel said.

“Huh?”

Allel pointed at the Shell above them. “Look. We must have fallen back. If we’d reached the Shell we’d see the world up there, a ball of rock, cupped by the Shell. And the land would tilt up at the horizon…”

Boyd grunted. Sensitive to her daughter’s mood, she drank in silence. She probed at her limbs. “At least we’re still whole,” she rumbled. She looked about. Then — unexpectedly — she grinned. “So we failed, did we? Eh?”

She dug her good hand into the ground, and then shook it in Allel’s face. “Look at that! Look!”

At the heart of the clump was a bright orange flower. A Shell flower.

Allel’s thoughts swam like fish. “Now I really don’t understand…”

“We made it. We’re on the Shell! That’s enough for me.” Then Boyd followed her daughter’s gaze upwards, to the roof over the world. Her eyes narrowed.

Allel said slowly, “Above us we see Home, not the Shell. Yet it looks as the Shell does. The two worlds are complete in themselves, yet they are — wrapped around each other. Symmetry. You see the same thing — a Shell — from whichever world you’re on.”

Boyd nodded shrewdly. “Well, that much I understand. Like us, eh? Two halves of the same whole. No weak center, no protecting Shell. Just the two of us.”

Allel dropped her eyes, hotly embarrassed. She went on doggedly: “But how? If we’re on the Shell, why doesn’t the land curve up like a saucer? Why don’t we see Home floating up there like a ball? How can it look like another Shell?”

Boyd made a little growling noise, and flung the shard of burner into the grass. A small flock of ice-blue birds clattered off, alarmed. “Well, you’re the dreamer. Dream up an answer.”

Allel lay flat. She rested her head on very ordinary loam and stared up through two layers of clouds. She thought of two worlds, each a ball yet each cupping the other like a shell round a nut. How could that be?

Her vision of her universe was crumbling, like the flaking planet-in-a-box milk painting on that museum wall. She imagined reaching into the box to the truth—

Boyd said gruffly: “Well, what now?”

Allel gestured vaguely. “Fix the balloon and get home. We’ve got to make people understand. Build more balloons and go to the old Cities. Find a way to turn back the glaciers, or fix the Sun…”

Boyd was staring past her shoulder. Allel turned — then sat up quickly.

The boy stood at the edge of the stand of cow-trees. He was no better dressed than they were; teeth flashed in a dark face as he jabbered at them, smiling and pointing and cupping his hands.

Allel watched, baffled. “What’s he saying?”

Boyd bellowed with laughter. “I think he’s asking what it’s like living in a saucer.”

Boyd stood up and, with some dignity, straightened the shreds of her quilted jacket. Allel got to her feet, stiffly. “Come on,” said Boyd. “Let’s see if his people can cook as well as your grandfather.”

They walked towards the boy across the meadow of bright orange flowers.

“Lethe. I can’t believe they fell so far. They’ve become utterly dependent on that artificial biosphere. They’re reduced to technologies of stone and wood—”

“But they survived,” Eve said. “Humans survived, even beyond the evacuation of the Xeelee. In a world that cared for them. You could argue this is a Utopian vision…”

“This world of theirs, with the Shell, is a four-dimensional sphere. No wonder they couldn’t figure it out.”

I thought of three-dimensional analogies. Allel’s people were like two-dimensional creatures, constrained to crawl over the surface of a three-dimensional globe. Home and Shell, the twin worlds, were like lines of latitude, above and below — each unbroken, each apparently cupping the other. Just as the diagrams in the “City” had tried to show them.

“But they were capable of understanding,” Eve said. “After a million years, humans had adapted in subtle ways. Allel had the capacity to visualize, to think in higher dimensions. She could have understood, if someone had explained it to her. As those diagrams in the place she called the City were meant to. And in time, she would figure out some of it…”

“They were trapped,” I said. “In a prison of folded space-time.”

“Perhaps,” said Eve. “Perhaps. But they didn’t give up…”