“No, I’m just calling it that. I mean, we won’t forget it if it has a name, right?”

Tamara furrowed her brow, considering. “I guess.”

“Better than Butterfly Pool,” Call said. “I mean, what kind of name is that for a lake that helps make weapons? It should be called Killer Lake. Or the Pond of Stabbings. Or Murder Puddle.”

“Yeah,” Tamara said drily. “And we can start calling you Master Obvious.”

The next chamber had thick stalactites, white as giant shark’s teeth, clumped together as though they might really be attached to the jaw of some long-buried monster. Passing beneath this scarily sharp overhang, Call, Aaron, and Tamara next walked through a narrow, circular opening. Here, the rock was pocked with cave formations that looked eaten away, as though they were in some kind of oversize termite nest. Call concentrated and a crystal in the far corner began to glow, so they wouldn’t forget they’d been this way.

“Is this place on the map?” he asked.

Aaron squinted. “Yeah. In fact, we’re almost there. Just one room to the south …” He disappeared through a dark archway, then reappeared a moment later, flushed with victory. “Found it!”

Tamara and Call crowded in after him. For a moment, they were silent. Even after seeing all sorts of spectacular underground rooms, including the Library and the Gallery, Call knew he was seeing something special. From a gap high in one wall, a torrent of water poured out, splashing down into a huge pool that glowed blue, as if lit from the inside. The walls were feathery with bright green lichen, and the contrast of the green and the blue made Call feel as if he were standing inside a huge marble. The air was redolent with the odor of some unfamiliar and tantalizing spice.

“Huh,” Aaron said after a few minutes. “It is kind of weird that it’s called the Butterfly Pool.”

Tamara walked up to the edge. “I think that’s because the water is the color of those blue butterflies — what are they called?”

“Blue monarchs,” Call said. His father had always been a fan of butterflies. He had a whole collection of them, pinned under glass over his desk.

Tamara put her hand out. The pool shuddered, and a sphere of water rose up from it. Even as it shifted and rippled across the surface, it kept its shape.

“There,” Tamara said, a little breathlessly.

“Great,” said Aaron. “How long do you think you can hold it?”

“I don’t know.” She tossed back a thick dark braid, trying not to let any strain show on her face. “I’ll tell you when my concentration starts to give.”

Aaron nodded, smoothing the map out against one of the damp walls. “Now we just need to find our way —”

At that moment, the map in his hands burst into flames.

Aaron yelled and pulled his fingers away from the blackening pages sparking through the air. The pages fell in a shower of embers and hit the floor. Tamara yelped, losing her focus. The water she’d been suspending splashed down over her uniform and turned into a puddle at their feet.

The three of them looked at one another, wide-eyed. Call straightened his shoulders. “I guess that’s what Master Rufus meant,” he said. “We’re supposed to follow our lighted stones or marks or whatever in order to get back. That map was only good for the way here.”

“Should be easy,” Tamara said. “I mean, I only lit one of them, but you guys lit more, right?”

“I lit one, too,” said Call, looking hopefully in Aaron’s direction. Aaron didn’t look back.

Tamara frowned. “Ugh, fine. We’ll figure out the way back. You carry the water.”

With a shrug, Call went over to the lake and concentrated on shaping a ball. Call drew on the air around him to move the water and felt the push-pull of the elements inside him. He wasn’t as good at it as Tamara, but he did okay. His ball dripped only a little as it hovered.

Aaron frowned and pointed. “We came in there. This way. I think …”

Tamara followed Aaron, and Call went after her, the ball of water spinning over his head as if he had his own personal storm cloud. The next room was familiar: the underground stream, the colorful mushrooms. Call navigated among them carefully, afraid that at any moment his water ball would fall directly on his head.

“Look,” Tamara was saying. “There’s lit-up stones over here….”

“I think those are just bioluminescence,” Aaron said in a worried voice. He tapped at them and then turned back to her with a shrug. “I don’t know.”

“Well, I do. We go this way.” She set off with a determined stride. Call followed, left-right-left, through a cavern full of huge stalactites growing in the shapes of leaves, don’t drop the water, around a corner, through a gap between boulders, keep it together, Call. There were sharp rocks all around and Call nearly walked right into a wall because Tamara and Aaron had stopped dead. They were arguing.

“I told you it was just glowing lichen,” Aaron said, clearly frustrated. They were in a large room with a stone cistern in the center, bubbling gently. “Now we’re lost.”

“Well, if you’d remembered to light up stones as we went —”

“I was reading the map,” Aaron said, exasperated. In a way, Call thought, it was kind of nice to know that Aaron could get irritated and unreasonable. Then Aaron and Tamara turned to glare at Call, and Call nearly dropped the spinning globe he’d been balancing. Aaron had to throw out a hand to stabilize the water. It hovered in the air between them, shedding droplets.

“What?” Call said.

“Well, do you have any idea where we are?” said Tamara.

“No,” Call admitted, glancing around at the smooth walls. “But there must be some way to find our way back. Master Rufus wouldn’t just send us down here to get lost and die.”

“That’s pretty optimistic, coming from you,” said Tamara.

“Funny.” Call made a face to show her exactly how funny it wasn’t.

“Stop it, both of you,” Aaron said. “Arguing isn’t going to get us anywhere.”

“Well, following you is going to get us somewhere,” Call said. “And that somewhere is about as far away as you can get from where we need to be.”

Aaron shook his head, disappointed. “Why do you have to be such a jerk?” he asked Call.

“Because you never are,” Call told him staunchly. “I have to be a jerk for both of us.”

Tamara sighed and then, after a moment, laughed. “Can we admit that we’re all at fault? We all messed up.”

Aaron looked like he didn’t want to admit it, but finally he nodded. “Yeah, I forgot that we weren’t allowed to use the map on the way back.”

“Yeah,” said Call. “Me, too. Sorry. Aren’t you good at finding paths, Tamara? What about all that tapping into the metal of the earth stuff?”

“I can try,” Tamara said, her voice a little hollow. “But that just lets me know which way north is, not how these passageways intersect. But we’ve got to come across something familiar eventually, right?”

It was scary to think about wandering through the tunnels, to think about the pits of darkness they could fall into, the sucking mud pools and the weird choking steam rising from them. But Call didn’t have a better plan. “Okay,” he said.