Discreetly, without making it too obvious, he leaned forward so that he slid out toward the edge of the group, away from Dura. And at the edge that strange new flavor of fear — a feeling of exposure — assailed him again. Shaking his head as if to clear out musty Air, he forced himself to turn away from the group, twisting in the Air, so that he faced outward, across the Mantle.

Farr knew that the Mantle was tens of millions of mansheights deep. But humans could survive only in a band about two million mansheights thick. Farr knew why... or some of it anyway. The complex compounds of heavy tin nuclei which composed his body (so his father had explained earnestly) could remain stable — remain bonded by exchanges of neutron pairs — only within this layer. It was all to do with neutron density: too far up and there weren't enough neutrons to allow the complex bonding between nuclei; too far down, in the cloying underMantle, there were too many neutrons — in the underMantle the very nuclei which composed his body would begin to dissolve, liquefying at last into smooth neutron liquid.

And here — close to the treetops, nearing the top of the habitable band — he was tens of thousands of mansheights above the site of the ruined Net.

Farr looked down, beyond his Waving feet, back the way he had climbed. The vortex lines crossed the enormous sky, hundreds of them in a rigid parallel array of blue-white streaks which melted into misty vanishing-points to left and right. The lines blurred below him, the distance between them foreshortening until the lines melted into a textured blue haze above the Quantum Sea. The Sea itself was a purple bruise below the vortex lines, its surface mist-shrouded and deadly.

...And the surface of the Sea curved downward.

Farr had to suppress a yell by gulping, hard. He looked again at the Sea and saw how it fell subtly away in every direction; there seemed no doubt that he was looking down at a huge sphere. Even the vortex lines dipped slightly as they arced away, converging, toward the horizons of the Sea. It was as if they were a cage which encased the Sea.

Farr had grown up knowing that the world — the Star — was a multilayered ball, a neutron star. The Crust was the outer surface of the ball, with the Quantum Sea forming an impenetrable center; the Mantle, including the levels inhabited by humans, was a layer inside the ball filled with Air. But it was one thing to know such a fact; it was quite another to see it with your own eyes.