-- and stared in astonishment at the far wall of the chamber, at least a hundred paces away. She stumbled backwards and landed in the snow, which soaked into her thin trousers. Boyd laughed, not unkindly, and hauled her to her feet. "A vast hall crumpled into a tiny hut. The people who built this had powers even you never imagined, eh?"

Allel stumbled around the tiny building. Where was all that space stored? If not sideways -- or behind -- or up, or down -- what fourth direction was there? The puzzle settled behind her eyes like a spider.

The floor area was empty, but the paper-thin walls were covered with pictures, still lit and animated after uncounted generations. "The pictures tell our story," Boyd said gruffly. "How we rose and fell." She stamped snow from her sandals and led the way around the walls. Afterwards Allel thought they could have walked in the opposite direction and lost little of the sense, for the story of humankind had a symmetrical design.

The bright side of the symmetry was expansion. From a world without a Shell, tiny ships like streamlined fish swam out on hyperdrive to the stars...

"What was 'hyperdrive'? And 'stars'?"

They were just words, Boyd said, passed on by other mothers on other days. Allel wondered if her balloon had risen on hyperdrive. She looked closely at the ships but could see no sign of burners. She tried to touch the picture--

-- and her hand passed into the depthless wall, in a direction she could not identify. She fingered a model ship; it was like a nut drawn on an invisible string. More mysteries...

At its peak humanity was a master of many stars -- which were evidently places very far away. And then--

"And then we met the Xeelee," said Boyd, and they inspected a harrowing battle scene. Elusive fingers snatched at the little ships. "Whoever they were, they were too big for us."

After the Xeelee wars came the dark obverse of humanity's conquest of the stars: its sad subsidence back to its home world, prodded by the dark fingers of the Xeelee.