“Ulick said I have to listen to it under the river.” But that was easier said than done. The river’s current roared past. Barclay would need to hold on to something so as not to be swept away.

“This better work,” Viola muttered. “All this salt water has been a nightmare for my hair.”

Barclay squeezed her hand as he lowered himself into the river. At first the current was so strong that he thought he would be torn out of Viola’s grasp. But then Viola gripped his arm with two hands—her nails digging painfully into his skin—and he submerged his head beneath the mucky water.

Whrrrrrrrrl.

The whooshing was louder than when the shell was simply in the ocean. Barclay turned from side to side, trying to judge which direction the sound was coming from.

Then he hoisted himself out of the river and back onto Mar-Mar.

“It’s toward the reef,” he said. “But the river forks ahead, remember? If we want to follow the sound, then I need to stay in the water.”

“We’re not rescuing you if you fall down another trench,” Tadg said flatly.

Viola sighed. “Then we’ll all ride in the river.”

“But we don’t have any Sunboards this time,” Barclay said.

“I have an idea.” Viola swam a few yards away and yanked out a long rope of seaweed from the ocean floor. She knotted it around her waist and handed the boys more to do the same. “Here. We’ll tie ourselves together so we don’t get separated.”

Barclay took it, grimacing at the seaweed’s slippery texture. Once they’d fastened themselves together, they floated along the riverbank, and Tadg returned Mar-Mar to his Mark.

“I’ll jump first,” Tadg said.

“No,” Viola told him. “We all need to jump at the same—”

Too late—Tadg leapt into the river, dragging Barclay and Viola with him. The current seized them and jetted them ahead. They clung to one another, all of them fighting and flailing to keep themselves upright and not spin out of control. Sand and pebbles peppered their skin, stinging like wasps.

“There!” Viola tried to raise her arm to point ahead, but she couldn’t lift it from the water. “That’s the first fork.”

Barclay ducked his head below the surface, listening to the whrrlllllll, then he lurched back up.

“Right! Go right!” he shouted.

They continued this over several more bends. The underwater rivers were a labyrinth, and Barclay wouldn’t be surprised if they stretched like a spiderweb across the entire seafloor.

The already fierce current grew fiercer. Barclay’s head bobbed in and out of the river. Even though he could still breathe, thanks to the Polypop mask, the speed made him dizzy. He wished he hadn’t drunk so much sour milk at supper.

“There’s another bend up ahead,” said Tadg.