“Do thee remember the crevice splitting the east wall, that we passed the previous day?”

“No. I was too busy looking over the bow to do any sightseeing.”

“I was working on my notes,” Lyra added.

“I have been watching the Barshajagad, marveling long, but always watching. There was a trail along the east wall. It terminates at this crevice. I have been this way once before. I saw no reason to mention the trail, to distract thee from thy work. We were to travel by boat, not by foot. But now I see that I must mention this other possibility or all will end.”

“A trail? I didn’t … no, wait a minute,” Lyra murmured. “I did see something. I thought it was an old high-water. Une.”

“It is a trail, Learned Lyra. A trading trail.”

“Where does it go?” she asked him.

“Up. Up to a high plateau. Not the Guntali. Higher than Turput but lower than that. High enough so that Tsla can live upon it: Up above the Topapasirut. The trail is steeper and more dangerous than the one that climbs to Turput.”

“How much more dangerous?”

“Enough to restrict travel. But it rises beyond the Topapasirut.”

Etienne gestured ahead, at the narrow, impossibly deep canyon. “I thought that this place was the Topapasirut.”

Tyl executed a gesture full of amused negativity. “No. If you would take the measure of the adversary that still lies ahead, thee must climb beyond its reach. That is, if thee think thy spirit boat can go no farther.”

“Point of diminishing returns, Etienne,” Lyra reminded him. “Every hour we’re using more power to cover less distance. Can’t you calculate how long it will be before we reach the break-even point and find ourselves making no progress even at full power?”

“Soon enough. You say there’s a way to get around this Topapasirut, Tyl?”

“Above, yes. Beyond, I do not know. I have not been to that place. But I have gazed on the Topapasirut. If thee would do likewise, thee must leave thy boat behind, at least until thee see for thyselves.”

“Leave it where?”

“Turn back to the trail end. I would not have thought there was a place, but thy boat can rise like a bird from the water.”

Etienne looked at Lyra.

“It’s your decision, Etienne. You’re the geologist.”

“Hydrology’s not my specialty, though. But it’s starting to look more and more like we’ve come to a dead end, at least as far as this section of the river is concerned.” He looked back at Tyl. “The east wall, you said?”

The Tsla nodded. Etienne turned back to the instruments. Unwilling to risk turning the boat broadside to the current, he lifted it on repellers and turned it neatly in midair before setting it gently back into the water.

There was a sharp lurch as the river caught the hull. He used only enough power to maneuver, letting the cells recharge as they raced back downstream.

“This is our best chance, Etienne. We have to see what we’re up against.”

“I know that, dammit. The canyon has to widen out somewhere above this. It can’t be like this all the way to the arctic line. Too much erosion. There’s a geological anomaly somewhere ahead of us.”

“And it has a name, apparently. Could it be another big waterfall, like the Cuparaggai above Turput?”

“No. I’m sure of that much, anyway. The water here isn’t acting like that and there’s no indication of a sharp rise in elevation at this point. The photogrammetrics are solid enough on that score. That’s why I don’t understand this intensified flow. But if this Topapasirut’s only the narrowest part of the canyon, we’ll see how much farther Upriver it lies. Maybe we can pass above it on repellers.”

By midday they had returned to the section of cliff Tyl had spotted. Etienne again lifted the hydrofoil on its repellers, set it down on an exposed beach out of reach of the river’s grasp.

The crevice to which Tyl had alluded turned out to be much more than that—it was a break in the Barshajagad wall, a sizable side canyon rising toward the sky. And the winding line against the rocks that Lyra had thought was a high-water mark did indeed show signs of use. It snaked along the granitic wall and started up the side passage.

“Now we climb,” said Tyl confidently.

“For how long?”

“Several days, at least.” He turned his gaze upward.

“Homat won’t be glad to hear that,” Etienne murmured.

“Why not leave him to watch over the boat?” Lyra suggested.

“Sensible. We’ll leave one of the porters with him and carry our own supplies. I don’t imagine he’ll regret missing the chance to view the ‘birthplace of all river devils.’”

“I will come with thee and leave the others behind, for I do not share thy trust of the Mai,” Tyl said.

“Homat’s been invaluable,” Etienne replied. “Without his aid we wouldn’t be here now.”

“His kind are not to be trusted.”

“In this instance I think you’re wrong, Tyl,” Lyra said. It was the first time Etienne had seen her openly dispute the Tsla’s opinion.

Tyl responded with a gesture of indifference. “Then I will direct Swd to remain behind by himself. While recuperating, he will keep watch over thy property—and its other guard.”

A small stream trickled down the branch canyon, which was indeed steeper than anything they’d climbed thus far. As the meters dropped behind and below them, so did the temperature. The Redowls were obliged to wear their long-sleeved and long-legged attire while Tyl and the porters re-donned their capes and togas. Since they didn’t expect to find any villagers to trade with, they carried a full stock of rations.

They ascended to the cloud. At five thousand meters Lyra was having some trouble breathing, though this was due more to the unaccustomed exertion than the altitude. Due to the denser atmosphere, five thousand meters on Tslamaina was equivalent to thirty-five hundred meters or so on Terra.

As they crested the trail head, the clouds momentarily parted. Ahead rose the peak of a steep-sided mountain that towered above them and the nearby edge of the Guntali. As they rested, Etienne took a sighting on the peak.

“Eleven thousand meters, most of it frozen.”

“Aracunga,” Tyl said. Etienne noticed that all the Tsla now wore their sturdiest clothing. They stood some two thousand meters higher than Turput, at the upper limit of the Tsla ecological zone. They could climb higher still, but not comfortably.

After several days of climbing to the east, they set off northward. Etienne expected Tyl to continue in that direction, but he was wrong. On the second day they turned slightly to the west, and by that evening the Redowls could feel thunder again.

They expected another waterfall, perhaps one that plunged the full five thousand meters to the river below. But it was no waterfall Tyl led them toward. It was the Topapasirut.

Must be the father of all waterfalls, Etienne mused, still unconvinced by Tyl’s denials. By the fourth day the thunder had become so loud they could communicate only by signs.

The Redowls could tap out messages on their wrist computers, but the Tsla possessed no such wondrous devices and had to make their intentions known through gestures. It grew damp around them, the rocks treacherous and slick. Yet as they hiked now through the perpetual mist, the sky overhead remained clear.

Etienne searched in vain for signs of the expected cascade. When they finally reached the lip of the abyss, all was explained.

It was raining upward. Forced into a narrow throat of the Barshajagad, the entire volume of the Skar suddenly made a sharp and unexpected bend from south to west. As a result, the swiftly flowing river cannonaded into the north cliff face that formed the base of the mountain Aracunga, five thousand meters below their feet.