Dichronauts

by Greg Egan

Nothing toppled, or broke, or went sliding out of control. Seth got up off his stomach and tipped his head west so he could hold onto the stilt and see what he was doing. It was disconcerting to be kneeling on the taut fabric of the hull, and feel the whole structure flex as he shifted his body. Even in the balloon, staring down through thin air from twice the height of any mountain, he’d been able to rest his body on the solid wood of the basket.

  Andrei performed the same maneuver, a little less clumsily. The four stilts remained in place, keeping the boat level. The ten-legged land creature had become some kind of tentative, shore-dwelling water-lizard; the only question was whether it was capable of making the transition all the way into the river’s depths.

  Andrei and Sarah leaned away from the boat, taking some of the weight off Seth’s corner, as Seth forced the top of his pivoted stilt southward, sending the bottom scraping its way north across the rock. The wedge that comprised the foot of the stilt didn’t need a Walker’s ankle joint in order to remain in contact with the riverbed; the slope was so close to forty-five degrees that the main effect was to increase the area of contact. They repeated the exercise for each corner in turn, then performed the second stage: putting as much weight on the southern edge as possible, while Seth and Andrei tipped the stilts back to the north while trying not move their bases. Finally, Sarah and Raina did the same. The whole process was exhausting, but having one edge of the boat in the water seemed to provide enough extra support to make it viable, and the way the buoyant force adapted to the boat’s changing orientation had a stabilizing effect.

  When their heads rose above the point where the water began to level out, Seth was both gratified and a little daunted. The river was far wider than the narrow terrace they’d first encountered; Theo couldn’t even ping the northern bank. If the runners worked, there’d be nothing to stop them making the crossing, but in the meantime the current would be carrying them east.

  “Maybe we should rethink our plans,” Sarah suggested. “If the water wants to take us this way, why fight it? We’re just as likely to find the edge of the chasm to the east as to the west.”

  Seth wasn’t sure how serious she was. Raina said, “Let’s just see what we’re dealing with. It’s still possible that we can get to the far shore without too much backtracking.”

  When the northern half of the boat was sitting almost level in the water, Raina and Sarah withdrew their stilts. Seth and Andrei gave one last push from the south, and suddenly the vessel was floating. The current took hold of it, and they began drifting eastward.

  Seth untied his stilt and collapsed it, then secured it at the bottom of the boat. To the west, in the distance, he could just make out the turmoil where the changing slope turned the north-flowing branch back on itself—but here the surface was almost flat, with none of the frantic activity of the overspill. After so much effort and anxiety, it felt glorious to be held up by the water and lofted along, whatever the direction, no longer having to worry about putting a foot wrong.

  Raina must have felt the same, because she gave them all a few minutes of relaxation before saying, “Now we need to deploy the runners.”

  The boards were already attached to the corners of the boat, but they were sitting horizontally, with no effect. At Raina’s count, the four Walkers in the corner modules tipped them, gradually and simultaneously, until they were sloping upward to the south at a modest angle.

  Water flowed up over the runners and spilled off the top, giving the boat a gentle push to the north—along with a gentle push downward, making it ride lower in the water. Seth peered into Theo’s view, but the far side of the river was still unpingably distant. The runners weren’t robust enough, nor the hull deep enough, for the boat to be driven much faster. And if there was nothing they could do to speed up the crossing, perhaps Sarah was right: perhaps they should welcome the effortless passage east, and just reverse the expedition’s original plan.

  The runners needed to be watched carefully, in case they snapped or came loose, and the imperfect mounts meant that they required small adjustments every few minutes, but compared to the arduous demands of the slope, to Seth the task felt almost shamefully effortless. «It looks as if I’ve finally joined you in retirement,» he told Theo, dipping a hand languorously in the water.

  «My retirement’s canceled,» Theo replied glumly. «When we get back, we’ll need to join the balloon project. It might be insanely dangerous, but if it’s the only hope then we have to be a part of it.»

  Seth was impressed that Theo had accepted Andrei’s word on the sun trap, and was even willing to reconsider the merits of a rival endeavor, but it was too much to hope that he’d change his mind about three things at once. «And if this river carries us to the eastern edge of the chasm, by nightfall . . .?»

  «That’s wishful thinking. There’s not going to be any eastern edge—or western edge, or southern edge.»

  Seth gazed out across the sparkling water. «If there’s nothing to the south of us but the void, what’s the source of this river?»

  Theo hesitated. «Rain clouds blowing in from the southern hyperboloid.»

  «Wouldn’t all the water there be frozen solid? If there’s no sunlight, ever?»

  «At the edge, maybe the summer air can circulate around and come back with more moisture than it started with.»

  Seth didn’t know what to make of this idea, but he couldn’t think of anything that ruled it out immediately.

  «We should be patient,» he decided. «The only thing I’m sure of is that we’ll have much more of a chance to learn the truth here than if we’d stayed up on the surface arguing about everyone’s theories.»

  night came, with no end in sight to the river, either to the north or to the east. Raina decided that they should withdraw the runners until morning; though the supervision they needed was minimal, it couldn’t be performed by anything less than a crew of four, and having each person sleep for just one-fifth of the night would have rendered them all incompetent for the day ahead.

  With no camp to set up, Seth felt more idle than ever. The only downside of their new accommodation was the disposal of bodily wastes, which had to be done via a container—and in Ada’s case, passed to a neighbor to tip over the side. Seth and Theo took the first watch, and with the boat no longer propelling itself, they pinged the water to the north with little expectation of change.

  «I hate not knowing where we are,» Theo said. Since they’d lost sight of the southern shore, they’d had no reference points to observe, so even their crude system of additive navigation had come to an end, replaced by even cruder estimates about the current.

  «We’ll make new measurements on the way back,» Seth replied. «We know more or less how far north of the basket we were when we entered the water, and however we return it will probably mean crossing this river in the opposite direction. And even if we don’t retrace our whole route, we ought to be able to locate that first terrace again, coming at it from the east.»

  When he judged his shift to be over, he woke Andrei and Nicholas then lay down to sleep. He couldn’t quite stretch out flat on his back, but with his head propped against the western rim of the hull, his knees bent and his feet against the eastern wall of his compartment, he felt more comfortable, and safer, than he’d ever felt in a sling.

  Someone prodded him awake. Raina said, “Seth?” It was still dark, but she wasn’t whispering.

  “What’s happening?” he asked. Theo woke, letting him see enough of Ada and Sarah to make it clear that everyone in the boat had been roused.

  “There’s a change in course for the river up ahead,” Amina said. Seth couldn’t hear anything, but Theo said, «I think she’s right.» The flooded terrace had had to come to an end eventually, but it hadn’t seemed too much to hope that they’d make it to the north bank first.

  “We need to be ready to reconfigure the boat,” Raina added.

  “Of course.” Seth’s grogginess was entirely gone now. Whichever way the rive
r turned, once it was no longer flowing over the terrace the surface of the water would take on more or less the same inclination as the underlying rock. The modules had been designed to allow for that: the southern pair could slide down lower than the others, allowing each individual part to stay level even on a forty-five-degree slope. But he’d always imagined making the modification in daylight, when he could see exactly what changes in the water were ahead, and how fast they were approaching. As close as Theo could ping to the north-east and south-east, there was still no sign of whatever it was the Siders were hearing.

  Seth loosened the ties around the sliders at the northern wall of his compartment, as much as he dared; he didn’t want to be fumbling to free them at the last moment, but if they were rattling about for too long they might become damaged.

  “We need a signal,” Andrei suggested. “To be sure we both use the sliders at the same time.”

  “Shouting ‘now!’ ought to do it,” Seth replied, still gauging the tension in the ties.

  “If you like,” Andrei said. “I was going to suggest that we treat any loud imprecation as proof that the other person has noticed the boat tipping, since that might incur less of a delay than anything requiring a conscious choice.”

  “It might,” Seth conceded. “But you don’t want to start reconfiguring the boat just because I stub my toe.”

  Ada said, “The river turns south. I can hear it now: there’s a clear asymmetry.”

  Seth didn’t reply, and no one else spoke to fill the silence. Was she claiming superior auditory skills of her own, or was she claiming that her drugged Sider was sharing its sense of hearing with her? Either way, he’d reserve his trust for Siders he could actually converse with.

  The boat glided on across the dark river. Theo showed the patches he was pinging shimmering slightly, ruffled by the breeze, but the surface was as inscrutable as ever.

  Then Sarah said, “There’s a cross-current, and it’s growing stronger. We’re accelerating south! Can’t you feel it?”

  Seth couldn’t honestly say that he did: he hadn’t been paying so much attention to the exact amount of pressure exerted on his body by different parts of the hull’s fabric that he could register any change now. But if they were coming to the end of the terrace, the direction of the flow might start to change well in advance.

  “What if we use the runners to try to get back to the south bank?” he suggested. “If the current alone would take us south and down the slope, an extra push might get us back on land before the river changes course.”

  Raina considered this. “All right. We’re probably too far north, but we’ve got nothing to lose by trying.”

  They had to redeploy the runners by touch, but the mounts gave a click at every increment in the angle, and the boat responded with a burst of speed that Seth had no trouble feeling. He moved his hand from the runner back to the slider, checking that it hadn’t come loose.

  Now that they had a chance to avoid the rush downslope, Seth found himself picturing the dangers far more vividly than when it had seemed inevitable. Even if the south-flowing river was deep enough to spare them from any rocks, when it came to an end on another flooded terrace, the transition was unlikely to be gentle.

  “How close now, Amina?” he asked.

  “A few more minutes,” she replied.

  Theo’s view still showed a level surface, but the water was growing choppy, with the usual orderly ranks of wind-blown waves broken up by more chaotic formations coming in from the east. Seth could see the boat’s wake too, imprinted on these natural undulations, but it looked feeble in comparison.

  Suddenly, an edge appeared to the south: a line beyond which the crumpled sheet of waves dropped out of sight. Seth shouted with jubilation; they were approaching the riverbank. He groped down and found his stilt. “We need to fix these again, don’t we?” he asked Raina. If they shot out of the water back onto the slope, they’d need some way to try to keep the boat level.

  She hesitated. “Yes.”

  Seth reattached the stilt to the side of the hull, while Andrei did the same.

  More and more of the shimmering surface Theo was painting gave way to darkness. Seth’s blood pounded in his ears and his body tensed, ready for the shock that would come through the stilts if they managed to scrape back onto dry land. In the complex topography south of the bank, there had to be a line where the slope increased, from a fraction less than forty-five degrees, to a fraction more. All they had to do was part company with the water on the near side of that line, where the slope would still bear them up and contain the river.

  At its eastern edge, the border of the encroaching darkness began to veer south, exposing a turbulent froth that was scattering pings in every direction. Seth was confused for a moment, then he understood what Theo was showing him: water from the terrace was arching down over the slope, rather than conforming instantly to the shape of the underlying rock and dropping neatly out of sight. If they failed to reach the shore, the boat would be riding that waterfall.

  “Reverse the runners!” Judith pleaded.

  “Why?” Nicholas asked.

  “We’re going down the slope, whatever we do. But if we travel farther east, if we can get past that corner, the flow should be smoother.”

  Raina said firmly, “No. Leave the runners as they are. We could still make landfall.”

  Seth watched the clean, sharp precipice to the south growing nearer—and growing shorter even faster. He wasn’t sure if Raina believed what she’d said, or if she’d reasoned that Judith’s tactic would come too late to make any difference.

  Theo inspoke calmly, «Five. Four. Three. Two. One.»

  The shoreline was behind them. Seth bellowed, “Now!” and pushed down on the slider’s handle as the boat lurched into the dark air. A fine spray stung his skin, and all he saw from Theo was the same scattering of droplets—and the northern half of the boat rising up to his left.

  They hit the surface of the southbound river, sank down almost to the top of the hull, then bounced up again. For a moment water rained down on them—but then the falls were gone, receding to the north so rapidly that they’d vanished behind the upper section of the boat before Seth could brace himself for a pummeling. A mixture of terror and a visceral sense of elemental power took hold of him as the boat accelerated in the current, easily outrunning any kind of rain. Gravity in midair was nothing compared to the forces on the slope itself, where the gradient rendered the efforts of the rock to retain its integrity beneath the modest weight of the river so stupendously inefficient that most of its resistance to compression was spent on directing the flow ever faster downhill.

  “Everyone still with us?” Raina shouted. Between the darkness and the boat’s new two-tiered shape Seth was in no position to check, but a chorus of affirmations came back. “Then hang the fuck on tight. When we hit the next terrace, we won’t have any warning.”

  Their descent seemed to have reached its terminal velocity, and while they bobbed and jittered in the water, making the hull shudder, it was only the force of the air rushing over the boat that kept them from moving in lockstep with the flow. When Seth cautiously dipped his hand into the water he could feel it vigorously outpacing them—but if he’d tried the same thing from the river’s shore, it would probably have torn his hand off. As it was, the wind bit into his skin as fiercely as anything short of a cone storm. Whatever change finally brought them to a halt, it was going to be painful.

  Ada said, “The air pressure’s up by almost fifteen percent.”

  “Could that be an artifact of the wind?” Nicholas suggested.

  “It’s an average, as much downwind as upwind.”

  “Then what does it mean, exactly?” Sarah asked.

  “That we’re lower now than we’ve ever been. Much lower.”

  Seth stared into the darkness. With every minute that passed, they might easily be traveling so far that it would take the expedition a whole day of struggling up the slope to
regain the lost altitude.

  Theo said, «We’ll find a north-flowing river.» That wasn’t an idle hope: as far as they knew, every drop of water around them had once flowed north. The more immediate problem was the end of their descent—and there was nothing to be done about that except to brace for it.

  The current around them grew more turbulent, shaking the boat, snapping rods and ties. The wind rose and fell in cheek-numbing gusts, so intense that they could only have come from dips and rises in the slope. Seth willed the torture of waiting to be over, imagining a dozen survivable scenarios, then imagining the worst and telling himself that he was ready for anything. But the river flowed on relentlessly, bearing them south like an idiot child who’d snatched something valuable from the adult world by mistake, fleeing ever faster out of sheer stubbornness and perversity. Having chanced upon the only living things in sight, all it could do was carry them deeper, farther from home, to no purpose, until it finally lost interest and smashed them against the rocks.

  when the sky began to brighten, Seth wondered if he was hallucinating: that their fall could outlast the night was no longer beyond imagining, but the intensity of the light far exceeded anything he’d seen since they’d left the surface. It took him a while to clear his head and make sense of the change: the shadow of the cliffs wasn’t infinite, and the farther south they went, the narrower the gap would be between the slope and the sunlit air above.

  As the river emerged from night, the water rushing past the boat glinted fiercely in the dawn. The boat’s own motion reduced the eastern bank to a blur, and it was only in the far distance that the rocks of the slope appeared still enough to perceive as anything solid at all. Seth searched the south-east corner of his vision for a river flowing from the west, then tipped his head and sought the opposite branch. But the landscape was almost indecipherable, and every distant shimmer that seemed like a portent of the boat’s imminent deceleration proved to be a false alarm.

  Amina called out, “I think—”

  Seth gripped the hand-holds beside him as the sky filled with water. Before he had a chance to take a breath, the boat had plunged so deep that every trace of light from above was gone. With Theo equally blind, Seth’s world shrank to the weight of the water crushing his chest, and the silence pressing in on him.