This produced a spray that rose on disturbed air to drench the puny observers clinging to a granite overhang. The solid bedrock trembled under the river’s impact. Tyl communicated with gestures, but any description was superfluous before the stupendous sight below.

Etienne knew that this was the Topapasirut, the birthplace of all river devils. He knew that Tyl had been more than right when he’d insisted no boat could pass through this place. The hydrofoil could not rise high enough on its repellers to clear the maelstrom.

Across the canyon, rising from the opposite side of the abyss, was a metamorphic mass that dwarfed even Aracunga.

“The Prompaj!” Tyl screamed into Etienne’s ear. He took another sighting.

“Fourteen thousand two hundred meters,” he informed Lyra via wrist computer. “An impossible mountain. I think the two peaks were once closer than now. See how the river bends sharply to the west before turning south again? Tslamaina’s seismically stable now, but a few eons back there must have been one hell of an earthquake in this part of the world. See the signs of slippage?” He pointed to particular strata down in the roaring canyon.

“This section of the surface slipped eastward. South of here the land went west. The result was the displacement of the northern third of the Skar several kilometers to the east. I’m glad I wasn’t around then.”

Lyra tapped out a reply. “I’m not real happy to be here now. Let’s get away. I’m cold and wet.”

They lingered a few moments longer so he could chip a few more pictures, take some final measurements. Then they headed back toward the trail head, leaving the clouds and hillsides to swallow up the Topapasirut, its thunder, and the brooding massif that was called Prompaj.

They made camp that night in a small cave, drying themselves and their clothes before a large fire. Etienne watched with interest as the porters groomed each other’s fur.

The Redowls said little. There was no point in belaboring the obvious. Their expedition had reached its end. They’d run up against not a brick wall but a watery one.

When the porters had finished and dressed themselves once more they gathered close around the warmth of the fire. Tyl spoke while his companions ate.

“What will thee do now, Learned Etienne? Does the spirit boat possess some magical power we have not seen that would enable it to pass through the Topapasirut?”

“It does not,” Etienne replied glumly. “We do have other machines which can fly through the air and put any bird to shame, but we don’t have one here. We chose to travel by boat. It’s all we have. You were right, Tyl. I apologize for doubting you.”

“You had not seen the Topapasirut, Etienne. No one believes until they have seen.”

“That’s it, then.” Lyra was not as disappointed as her husband, though she strove to sound as sympathetic as possible. If they could no longer go onward, they would have to go back, and she still had work to do among the Tsla.

“You’ve been stopped by a geological phenomenon, Etienne. What better way to conclude your report? Think of the reaction among your colleagues when you describe this place. Maybe someday we can come back up here with an aircar.”

He’d been staring at the floor of the cave. Now he looked up, determined. “They’ll be fascinated, but it won’t be the end of my report.”

“Etienne,” she said gently, “we can’t get through that chute. You’ve already acknowledged that.”

“I won’t be stopped by the very river I’ve come to survey.”

She sighed, leaned back against the inflated sleeping pad. “Maybe you’ll accept it by morning.”

“Maybe.”

He did not, nor did he admit defeat during the long descent to the Skar. He kept to himself and brooded, causing Tyl to move next to Lyra.

“What ails Etienne?”

“He’s unhappy because he knows we can’t go on. That means he’ll have to leave his work here unfinished.”

“But it is not his fault. Nothing passes Upriver beyond the Topapasirut. He has no control over that. It is not as if he were beaten by something in himself.”

“He knows all that, Tyl, but he is persistent, Etienne is. Always has been.”

“I see. A Tsla teacher would accept the inevitable; such constant worry is harmful to the mind.”

“True, but sometimes it can lead to solutions where none seem possible. I’ve seen him do it before. Within our fields, Etienne and I are well respected. We’ve achieved success where others have failed. It’s one of the reasons we were allowed to make this expedition while other applicants were rejected. Sometimes, Tyl, blind persistence can succeed where everything else has failed.”

“I still do not understand why you would sacrifice peace of mind. I can admire such tenacity, but I cannot empathize with it.”

Down on the river there was a brief but joyful reunion with those left behind. Homat didn’t try to conceal his relief over the safe return of his human protectors.

“All these days,” he whispered to Lyra later, “trapped with that Tsla, and him mumbling and chanting to himself all the time. It was enough to drive a sane person crazy. Did you find a way to pass this Topapasirut?”

“No, we did not.” To her surprise Homat looked downcast. “I thought you’d be pleased. That means we have to go back Downriver now, back to the warm lands of the Skatandah. Don’t you miss them?”

“Very much so, but I have joined myself to your purposes and therefore am disappointed for you.”

“That’s a very nice thing to say, Homat.” She hadn’t expected such depth of feeling from the Mai. Nor was it a ruse. He was genuinely distressed that their journey had come to an end.

She looked past him, frowned. Etienne was deep in discussion with Tyl and looking more animated than he had in many days. She strolled over to join them.

“What’s all the excitement about?”

“You tell her,” Etienne suggested to Tyl, his features alive with enthusiasm.

“On the eastern flank of Aracunga Mountain,” Tyl explained, “lies the Tsla trading town of Jakaie. I have not visited it myself but it is known to Turput. It is said that beyond Jakaie and the mass of the mountain, the Barshajagad once more becomes a navigable river. If thee could but convey thy craft to that place, thee might safely resume thy journey—if the story is accurate.”

“An impossible if.”

“Maybe not,” Etienne murmured. He was tense with possibilities. “Maybe we could portage around.”

For a long moment she just stared at him. Then she let her gaze trace the lower section of the steep trail that wound its torturous course up the side canyon.

“Sure we could. We’ll just hoist the boat onto our shoulders and haul it five thousand meters straight up. Lost your mind?”

Her skepticism didn’t even slow him down. “No, I’ve just found it. Look, the hydrofoil’s made of ultralight material. The hull’s a carbon filament honeycomb. And we can surmount the rough spots on the repellers.”

“With what power?” she argued. “We’d burn out the cells.”

“We would not. You’re not listening. We’d only use the repellers to get over real steep places. The rest of the time we’d rely on muscle power. Porters, Lyra! Mount the boat on some kind of platform and pull it up and over.”

She did some quick figuring. “I admit the hydrofoil’s light, but it’s a relative lightness. You’d still need a thousand Mai or Tsla to drag it up a thousand meters.”

He looked back at Tyl. “Tell her.”

“There is a draft animal,” the Tsla explained, “that the Mai use all along the river. It is called a vroqupii. The Mai use them in teams to pull trading boats Upriver against the current. They are strong.” He eyed Homat. “Well, Mai?”

The guide looked thoughtful. “We passed many trading villages below this place. Each should be home to a few vroqupii. The animals used hereabouts must be unusually powerful because the current is so fast.”

“Do you think we could find enough to do it?” Etienne asked.