The tree trunk she'd followed had broadened only gradually, at last reaching a width just too great for her to stretch her arms around. Now, suddenly, the clean lines of the trunk exploded into a complex tangle of roots which formed a semicircular platform over her head. Peering up, she could see the roots receding into the dim, translucent interior of the Crust itself; they looked almost like human arms, reaching deep into the gossamer solid in search of neutron-rich nuclei of molybdenum, strontium or krypton.

Looking around, she saw how the root system of the tree merged with those of its neighbors in the forest, so that a carpet of wood formed an impenetrable ceiling over the forest. A few strands of purplish grass sprouted among the roots. The tree trunks, following the Magfield lines, met the root ceiling at an oblique angle.

Soon the others had joined her. The four Human Beings huddled together, clinging to loose roots for stability. It was so dim now that Dura could barely make out the faces of her companions, the outlines of their thin bodies. Philas's eyes were dull with exhaustion and apathy; Farr, trembling, had his arms wrapped around himself, and his mouth was wide as he strained at the residual Air. Adda was as uncomplaining as ever, but his face was set and pale, and Dura could see how his old shoulders were hunched over his thin, heaving chest. Adda took leaves from the bulging pack at his waist. Dura bit into the food gratefully. Insubstantial and unsatisfying as it was, the food seemed to boost what was left of her strength. Farr continued to shiver; Dura put an arm around him and drew him closer to her, hoping to transmit enough of her body warmth to stop the trembling.

Farr asked, "Are we at the Crust?"

"No," Adda growled. "The true Crust is still millions of mansheights above us. But we've reached the roots; this is as far as we can go."

Philas's voice was low and harsh in the thin Air. "We can't stay here for long."

"We won't need to," Dura said. "But maybe we should open up a trunk and start some nuclear burning again, before we congeal here. Adda, could you..."

The old man raised a hand, curtly. "No time," he breathed. "Just listen... all of you."

Dura frowned but said nothing. The four fell into a silence broken only by the rattle of their uneven breaths. Dura felt small, vulnerable, isolated, dwarfed by the immensities of the root systems over their heads. Every instinct ordered her to bolt, to slide back down the tree and plummet through the wall of treetops to the open Air where she belonged; and she could see the same urges in the set faces of the others.

There. A rustle, a distant grunting... It came from the root systems, somewhere to her left.