The tetrahedral artifact loomed beyond the small-scale human activities. It was a skeletal framework, incongruous, sharp, dark.

Borz and the woman hung back as Mur and Philas hesitantly approached the tetrahedron's forbidding geometries. Mur peered up at the framework. The edges were poles a little thicker than his wrist, each about ten mansheights long. They were precisely machined of some dull, dark substance. The four triangular faces defined by the edges enclosed nothing but ordinary Air — in fact, the people here had slung sections of net to enclose a small herd of squabbling, starved-looking Air-pigs at the framework's geometric center. Elsewhere on the framework rough bags had been fixed by bits of rope; irregular bulges told Mur that the bags probably contained food, clothes and tools.

Mur moved forward, reached out a tentative hand and laid his palm against one edge. The material was smooth, hard and cold to the touch. Maybe this was the Corestuff of which Dura had spoken, extracted from the forbidding depths of the underMantle by City folk (and now, unimaginably, by the boy Farr whom Mur had grown up with).

Philas asked, "Can we go inside?"

The woman laughed. "Of course you can. Your friend was right... nothing works, any more."

The man grunted to Mur. "We'd hardly keep our pigs in there if they were going to be whisked off to the North Pole at any moment."

"I imagine not."

Philas passed cautiously through one face of the tetrahedron. Mur saw her shiver as she crossed the invisible plane marked by the edges. She hovered close to the pigs and turned in the Air, peering into the corners of the tetrahedron.

The man — Borz — grunted. "Oh, what the hell." He dug into one of the bags dangling on the tetrahedral frame and extracted a handful of food. "Here."

Mur grabbed the food. It was stale, slightly stinking Air-pig flesh. Mur allowed himself one deep bite before stuffing the rest into his belt. "Thank you," he said around the mouthful of food. "I can see you've little to spare."

The woman drifted closer to him. "Once," she said slowly, "this frame sparkled blue-white. As if it was made of vortex lines. Can you imagine it? And it really was a wormhole Interface; you could pass through it and cross the Mantle in a heartbeat." For a moment she sounded sad — nostalgic for days she'd never seen — but now her dismissive expression returned. "So they say, anyway. But then the Core Wars came..."

After raising several generations of Human Beings, the Colonists had suddenly withdrawn. According to the Human Beings' fragmented oral histories the Colonists had retreated into the Core, taking most of the marvelous Ur-human technology with them, and destroying anything they were forced to leave behind.

The Human Beings had been left stranded in the Air, helpless, with no tools save their bare hands.