Грег Иган - Phoresis

She reached the bottom of the level just as night fell, and found the seals of the connecting chamber not just intact, but blocked by outgrowths of ice. She took the chisel from her pack, but then thought better of trying anything in the fading light. If she ruined the seals and lost all the air from another level, that would be unforgivable. So she spread her blanket on the floor beside the chamber, and slept.

Standing in the open air at the top of the final staircase, with the wind and dust of the ancestral world blowing in her face, Petra felt her brothers begin to stir.

The stairs she could see in front of her weren’t any more malformed and bulbous than the thousands of helical ones she’d already negotiated, but the unbroken line from the landing to the still distant ground did induce a new kind of vertigo: if she slipped and fell, it wasn’t clear how far she might descend before she came to a stop. The swollen ice on the side walls had spat out all the railings to which she might have hooked a safety rope; all she could do was keep her hands on the walls to steady herself, and take scrupulous care with where she placed her feet.

As she prepared to step off the landing, she felt her brothers rebel, all but paralyzing her with fear. She only had one more pessary left, and whatever progress her friends might have made on the rope bridge by the time she came back, she was fairly sure that her need to exercise unfettered judgment would be even greater then. “What would you have me do?” she muttered. “Stay here forever and starve to death?”

She still had food in her pack, of course. And whether her brothers were aware of that or not, she wasn’t really arguing with them, so much as with a kind of conspiracy they’d formed with her own, more ancient instincts. Ancient Petra certainly knew about the food.

Modern Petra took off her pack, pulled out the sack of dried meat and tanglers, and tossed it down the stairs. It came to a halt before it vanished from sight, and it did not appear to have split open. She hadn’t really intended the gesture as an experiment to test her own vulnerability, but the results were encouraging: even if she did fall, it might not be too terrible.

The conspiracy was appalled, but it was also hungry enough to be disinclined to walk away from food. Petra put her pack on and started making her way down, promising everyone that she’d start chewing on a tangler once she reached the sack—and hoping that it would seem easier and safer by then to continue down the stairs than it would to turn back.

The ice field around the base of the tower was desolate. If there had ever been tents or other buildings here, no trace of them remained.

Petra inhaled the thin, dusty air and contemplated the task ahead of her. The hemisphere visible through the telescopes contained no farms or grassland, and everything that wasn’t ice resembled outcrops of bare rock, or pits of gray dust. But if Tvíburi had proved anything, it was that a food source didn’t always look the way it was expected to look.

Evening was approaching, so she set up her tent in the partial shelter of one of the tripod legs. Whatever else had befallen Tvíbura, the ocean must still be thriving if the Yggdrasil that held the tower up hadn’t withered and died. If only the people of the surface could have found a way to live down there in the sweltering darkness, sheltered from the vagaries of the sun and the geysers, floating through the molten ice.

In the morning, she set off east, guided by the sun but also looking up often at Tvíburi to impress the meaning of its features on her so she’d have no trouble reading them, day or night. The idea that she might somehow fail to find her way back to the tower did not remain unthinkable for long; by the end of her first day walking, it had vanished from sight, and while the maps in her pack were as detailed as she could make them from Tvíburi, down on the ground, for stride after stride, the only phrase her surroundings evoked was: ice is ice .

On the second day, it rained heavily, and she filled a bottle with ethane. When she squatted down to inspect the rivulets running over the ground, she saw fine dust pinned by surface tension to the top of the liquid, but no matter how closely she squinted at the surface she could not make out a single struggling insect.

On the third day, she reached a cluster of hills. She clambered over the rocks, in and out of shallow, sheltered valleys, but they concealed nothing she could recognize as living.

On the fifth day, with Tvíburi low in the sky, Petra came to a place that was known to have been farmland. The topography had kept the degraded soil from dispersing across the whole surrounding ice field, but the swirls of gray dust through which she strode did not remain still enough to make a home for any conceivable form of vegetation.

The fields were long gone, but where the center of the village must have stood, she found the remains of half a dozen buildings, with stone or wooden foundations set in pits hewn into the underlying ice. Petra knelt down between the ruins, covering her face against the dust. At least some of the people of this village had reached Tvíburi safely; that much was recorded in the archives. But what would she tell their descendants when she returned? That there had been no invisible salvation, no revolution in Tvíburan agriculture that could keep people fed as the soil blew away. No one had any right to be surprised by that, but she could not blame them for hoping otherwise.

When she set out on the seventh day, Tvíburi hung bisected on the horizon behind her. Petra felt a sense of panic at the prospect of losing sight of her own world; once it vanished, she might find herself wandering endlessly beneath an empty sky. But she stared down her fears and kept walking. Every place where she’d set foot until now had been scrutinized for generations, and all that anyone had seen through their telescopes had been farm after farm dying. But half the world had remained hidden, and from the day the last migrants had alighted on the Tvíburian ice, no more news of its fate had been forthcoming. Many people had made the Great Walk on Tvíburi—and at least in the past, on Tvíbura too. If she wanted to return with a final answer on the fate of this world, the only way was to revive that tradition.

Petra didn’t expect the old maps of the far side to tell her much; when she’d compared migration-era maps of the visible hemisphere with current, telescopic versions, four out of every five villages had vanished entirely. But with nothing else to guide her, she sought out the places where people had once lived.

Sometimes it was difficult to know if she’d reached the right site and found nothing, or simply failed to pin down the correct location. Unlike farmland and grassland, hills and mountains didn’t blow away in the wind, but it was rare that they could serve as reliable signposts.

By her thirteenth day on Tvíbura, all her food had run out. She’d been expecting that, and as an abstract idea it didn’t worry her; all her training told her she could keep up her strength and complete the Great Walk in thirty days, at which point Rada would be waiting for her at the base of the tower with fresh supplies. But starving herself by choice, as an exercise, within half a day’s walk of her own village back home, was not the same as marching empty-handed over an endless expanse of barren ice on the most distant part of a world that for all she knew was entirely lifeless.