AURIEL IS AGITATED. His sunshine-yellow color has turned more mottled, like a fiery late afternoon sun stained through by the beginnings of sunset. Oranges and reds pass through him as he slithers and licks the air, circling the makeshift raft while the shadowwalker boys and their daywalker allies work on it. Rumi watches Auriel twine himself through the fibers and leaves and mud.

Gogi has been using his fire to harden the mud on the sides of the escape raft, to make them waterproof. He takes a break, though, collapsing against a palm tree at the edge of the beach. Rumi plops his way across to him, sitting on Gogi’s knee and using his magic to send breezes over the overheated capuchin.

“Thanks, buddy,” Gogi says. “I’m so glad you got your air power back. This cooling wind is a total life changer.”

“Have you noticed Auriel lately?” Rumi asks between gusts.

“You mean the weird colors and the frantic slithering? Yep, I noticed. Sort of hard not to.”

The air flashes red as Sky lands beside them. “Yes, I’ve been wondering about it, too. I’m not as concerned about the color as his change in behavior. Anxiety seems out of character for him.”

“I’m not sure what would count as in character for a magical boa constrictor who’s been brought back from the dead,” Gogi observes.

“Fair enough,” Rumi says.

“If only he could talk to us,” Sky says. “Then we might be able to know what’s on his mind. We’ll watch him closely. That’s all we can do. Oh, wait. Would you look at that!”

“Look at what—oh!” Rumi says. Auriel has threaded his way down to the water and is making little forays into the surf.

“Can he even swim?” Gogi asks.

“I don’t know,” Rumi says. “I’ve seen many snakes swim before. I certainly hope Auriel can, since he’s going right into the water.”

“Should we be doing something?” Gogi asks. “In case he drowns?”

“I can swim!” Zuza the tapir calls from where she’s constructing the far side of the raft. “You might not think it to look at me, but I’m pretty good. Want me to go in and start a rescue?”

“That’s okay!” Gogi calls back. “You just keep on hauling lumber, please!”

“Okay!” she says cheerfully, the beach soon filling with the clamor of logs striking logs.

Auriel ventures into the surf, then looks pointedly back at the shadowwalkers before heading back into the surf again. Rumi sighs. “I think I know what this means. Auriel wants us to start our underwater adventure now.”

“I hadn’t thought Auriel would be coming with us down there,” Sky says.

“I’m actually sort of glad to have his company,” Gogi says. “For a reptile that once tried to kill me and now doesn’t say a word, I’m pretty fond of him. And apparently he’s fond of us.”

“I wouldn’t say he’s fond of us,” Sky says. “My guess is we’re useful to him, that’s all.”

“We can always trust you to have the most cynical interpretation,” Gogi says, rolling his eyes.

“Hello . . . everyone?” comes Banu’s voice from the far side of the craft. “Are you all seeing . . . that Auriel’s gone swimming?”

“Yes!” Rumi croaks. “Are you ready to go?”

“I’ve been practicing . . . my skills,” Banu says. “I think . . . I’m ready!”

“Not sure if ‘I think I’m ready’ is what I want to hear before diving into the sea,” Gogi grumbles.

“Imagine everything that we’ll discover!” Rumi says rapturously. “There could be all sorts of life down there, plants that we’ve never seen or heard of—”

“Monsters and beasts all ready to eat us,” Sky adds.

Gogi looks between them. “I’m feeling very complicated emotions about you two right now,” he says.

Rumi bounces along the hot beach, gasping whenever the sand sears his sensitive hands. He douses them in the water at the ocean’s edge, but the salt only makes the burns hurt more. While he debates whether to stick his hands back in, a wave bowls him over. He sputters. Now the salt is stinging his entire body. Sometimes it’s hard to be a frog, there’s simply no way around it.

Something strong grips Rumi’s body, then he’s lifted to safety. Rumi’s surprised to discover that it’s Sky’s beak that plucked him free, not Gogi’s hand or tail. Sky gently lays him out on a piece of dried seaweed, cooler than the surrounding sand. “Thanks!” Rumi says, worrying his hands as he stares into Sky’s mysterious black eye. “Sometimes I get too excited to start, and it gets me in danger.”

“I do not tend to get overexcited, but I can imagine what it might feel like,” Sky says. “I’m very happy to rescue you.”

Gogi’s standing in the seawater, looking brave and looking miserable. Rumi knows how much the capuchin hates getting wet, and right now he’s standing on his legs, arms awkwardly over his head, gasping and yelling each time the ocean water gets higher on his body. “Let’s hurry up,” he says. “I’m ready to be done and drying off already.”

“On it,” Banu says. Rumi looks around, trying to see where his friend is, unable to know for sure because of the roar of the waves. Then he sees it—Banu is swimming. He’s surprisingly agile, his long-clawed arms cutting cleanly through the waves. Banu closes his eyes for a moment, then suddenly he’s standing on the ocean bottom. The water has parted around him in a neat U, dark seafloor exposed all the way up to the beach, two crabs blinking in surprise at the sudden sunshine before scuttling off and disappearing into the wall of water.

“Wow,” Rumi says.

Banu looks around, claws on his hips, proud of himself. “Pretty nifty, huh?”

“Very!” Rumi says, clapping.

Gogi takes a step forward, then shrieks when his tender belly gets wet. “Could you move a little closer to shore?” he asks.

“Oh . . . of course,” Banu says. “Sorry.”

The sloth crawls toward the shore. Once the circle of open space joins the beach, Gogi steps in. “Ooh,” he says. “Wet sand feels very nice on blistered feet.”

Rumi hops to join him, and lets out a sigh of relief. “Agreed! Even though it’s a little too salty for my tastes.”

Auriel swims through the waves, easily surfing the ocean current before popping out of the wall of water to wriggle along the wet ground.

“Looks like Auriel’s decided he’s definitely coming with us,” Gogi says.

While Sky struts his way into the circular clearing, Rumi looks toward the line of palms that separate the beach from the jungle. “I’d hoped Mez and Lima might be back by now,” he says.

“They’ll be right here waiting for us when we return,” Gogi says. “I’m sure of it.” Rumi appreciates his friend’s hopeful words, but can’t help but notice the worry drawing the monkey’s lips tight.

Rumi creeps toward the wall of water, where a school of fish stares back at him, seemingly as astonished as Rumi is by this strange turn of events. Banu takes a step farther, which causes the open space of air to move too, surprising one of the fish. One moment it’s floating in the water, and the next it’s plopped to the ground, gasping. Rumi gets his sticky fingers around the fish’s glistening scales, then hurls it into the wall of water. It looks back, blinks a fishy thank-you, then speeds off into the sea.

“Are you all . . . ready?” Banu asks.

“We are,” Rumi says. “Just think of it—we’re about to uncover more mysteries of the two-leg civilization. I couldn’t be more ready!”

“Rumi’s got enough enthusiasm for the rest of us,” Gogi says, eyeing the wall of ocean water dubiously.

“If it still exists, that two-leg tunnel we saw in the carvings would pass right into the volcano beneath Caldera,” Sky says. “This could be the solution to all of our problems.”

“Yes, yes, I know,” Gogi says. “That doesn’t mean I have to be comfortable with going under the ocean.”

“Bye, Zuza!” Rumi calls.

“So long, guys, have fun!” Zuza responds from far up the beach.

“Not likely,” Gogi says, arms tight around his chest.

“Here we go,” Banu says. He crawls away from the beach, the bubble moving with him, centered around the sloth. The companions keep as close to Banu as possible. Rumi rides on his shoulder, Sky struts alongside, Gogi hobbles behind, so close to Banu that he’s clutching him hard around the backside, like a baby monkey riding its mother. Auriel is the only fearless one, patrolling the edges of the circle, alternating tastes of air and saltwater.

As the seafloor slopes down, the water appears to rise around them, soon towering high overhead. It begins to crawl into the air over them, water flowing and spreading so it finally meets right above Banu, so that they are traveling along the ocean floor in a perfect hemisphere of open space. It gets dark like a den, too, as the seawater above chokes out the sunlight. It’s dusky for a while, and then the light goes out entirely, as if the Veil has dropped.

Gogi readies a fire on his tail, like he’s done before when nightwalking. The air immediately turns smoky. Before anyone needs to tell him to, Gogi drops the flame. “Sorry, guys!”

It turns out to have been unnecessary. As it gets darker, Auriel brightens his sunshine yellows, giving the magical hemisphere of air the radiance of a cool dawn.

The seafloor beneath them changes, turning craggy. The rocks are slimy, often covered in green plants, and strange sharp white things that Rumi witnesses sometimes extending a little frond. “I wonder if they use those to catch their food,” Rumi ponders aloud. His friends are too distracted, though, to be much interested in his intellectual investigations.

It gets chillier and chillier as they go, and the companions find themselves huddling tighter together, not for safety as much as for warmth. Gogi adds just enough fire magic to his fur to heat and dry it, and the hemisphere fills with the pleasant, yeasty smell of drying animal.

Rumi wishes he could pause every few feet, to observe and examine and study. The seabed isn’t perfectly flat, for one thing, and not even all that sandy! The ground is hard, and covered with rocks and unfamiliar creatures. A spiny thing that looks like a giant cockroach stares back at them with eyes on the ends of stalks. A blobby creature tumbles to the ground when the wall of air hits it. Sky and Banu walk over it just fine with their claws, but when Gogi steps on it he howls in pain, sending Rumi tumbling down to the ground—luckily not onto the strange stinging creature. Once Banu passes by, the stinging blob lifts back up into the ocean, floating off into the watery dark.

“Fascinating,” Rumi says.

“Painful,” Gogi corrects.

At first Rumi looks forward to Banu’s frequent breaks, since he can further explore the rubbery plants of the seafloor. But as they continue down and Auriel’s eerie light feels chillier and chillier, even he wishes they could get on with it, so they could be back on land as soon as possible. There’s so much water above them now, enough to drown them many times over. Or maybe it would crush them before it could drown them? Hmm! Rumi does some fast calculations in his head. Yep, it would crush them first.

He shivers and draws even nearer Gogi’s fire-warmed fur. The tireless pursuit of knowledge definitely has its downsides.

The seafloor gets even rockier, then the rocks rise steeply enough that Banu, even with his long curved claws, can’t get a grip.

“Wait, everyone,” Sky says as he closes his black eyes. He opens them. “From my memory of the map in the Cave of Riddles, we’re entering the strange zone of vertical trees, or whatever is left of them. Once we’re over this blockade and through the vertical trees, we’ll get to the tunnel that heads into the side of Caldera, which might lead to the magma source.”

“Okay,” Rumi says, shivering as he peers upward, froggy hands on his hips. “We have to get over this rocky cliff, then, don’t we?”

“Let me give this one more shot,” Banu tells Rumi. “Wait down here until I’m sure I won’t fall.” After the tree frog hops off his back, Banu carefully selects two handholds, wedges his foreclaws into them, and pulls. Then he wedges his back claws into two cracks and heaves himself farther before selecting two spots for his foreclaws, higher up. “It’s . . . working!” he calls down excitedly. “I’m . . . climbing!”

Rumi stares up. “That’s great, Banu!” He flicks his back legs against a strange tickling sensation, but it’s right back, even stronger. He flicks again, but in response he’s suddenly lost in cold, dark sea. “Help!” he shouts, or tries to shout, only inhaling saltwater instead.

Then there’s a hand darting in from the darkness, gripping him around the midsection and pulling him back into the dryness and light. A monkey finger pushes his belly, and Rumi lets out a fountain of ocean water. Once his panic fades, Rumi’s expression settles on Gogi’s worried expression. “What’s the matter?” Rumi asks.

“You okay, buddy?” Gogi asks.

Rumi nods. “Water’s not as scary to a frog as it is to a monkey. But I’m still very glad you rescued me. Now, um, you should consider rescuing yourself.”

Gogi looks down and yelps when he sees that the dark seawater is up to his ribs, and continuing to rise. “Why is this happening?” he asks, thrusting Rumi over his head to keep him out of the water for as long as possible.

“Banu’s moving upward, and he’s bringing the water-free zone up with him!” Rumi says. “Hurry.”

“On it,” Gogi says. He’s a magnificent climber, scrambling his way up the slick rocks until he’s beside Banu, the water-air boundary safely below. They’re surrounded by cool, buttery, shifting yellows as Auriel snakes through the crannies alongside them.

“All present and accounted for?” Sky asks, flapping nearby, sending gusts of cold air over them as he flutter-lands against the wet kelpy rocks.

Rumi does a quick head count of the shadowwalkers in the bubble. “Yep! All present.” They’re not looking in great shape, though. Gogi’s breathing shallowly, eyes darting about in low-level terror; Sky’s feathers are wet, which must be impeding his ability to fly, not that there’s anywhere he can go; Rumi can feel his skin stinging everywhere from the salt—that mouthful of seawater didn’t exactly help, either; Banu’s eyes are half-lidded—he’s clearly getting tired from the expenditure of magic. Auriel’s the only one who seems unaffected by their strange journey. He weaves along the rocks, tasting the air, inscrutable.

After resting a moment, Banu lurches back into motion. “All right, Banu,” he encourages himself. “Up . . . and over!”

Gogi scrambles to keep up as Banu disappears over the crest of the rock, then scrambles even harder when Banu grunts and falls on the far side, taking the bubble of open space with him.

Hand over foot over tail, Gogi just manages to keep ahead of the wall of water behind them, bringing Rumi along for the tumbling, freewheeling ride. They land on a bed of kelp. Between and under the slippery leaves is a strange surface—it’s somehow both rough and soft, flecks of jet black within the rubble. Rumi runs his sensitive fingers and toes over it. “Organic composition, but a tightly bonded mixture that I’ve never encountered before. I wonder if this is something created by the two-legs, perhaps, as a way to make their vertical trees. Do you think we could stay here for just a little while, so I could determine more of this substance’s properties?”

There’s no answer from his companions. Their focus is trained upward.

A strange corner juts into the bubble, just at the edge of its circumference. It’s the edge of one of the two-legs’ vertical trees. Auriel seems especially intent on it, lacing his way forward along the seafloor, before he dips his head into the wall of water. His radiance passes through the membrane and into the ocean, lighting up the hemisphere surrounding them. Now they can see a few feet into the gloom of the surrounding depths, ominous shapes lurking in the dark.

“Are those . . .” Gogi asks.

“—those are the vertical trees from the carvings in the Cave of Riddles,” Sky announces. “We’re in the home range of the two-legs.”

“Right,” Gogi says, letting out a long breath. “I didn’t think those were tall monsters looming over us, nope. I’m super brave, we all know that, it’s all good.”

“Could you push the water back even farther?” Rumi asks Banu.

“There’s a lot . . . of pressure,” Banu says. “But since we’re not moving . . . I think I can manage it.” His expression scrunches as he reaches deep into his magic.

Slowly the radius of the bubble pushes back farther, and farther.

Once it’s gotten far enough, they all know precisely where they are.

They draw even closer together, awed into silence.