The old swimming pool behind Hinode Ridge High School had been drained for six years, and in all that time nobody had thought to take down the fence. That was how the Science Club had found it, back in April: a rectangle of cracked turquoise tile, a diving board bleached the color of old bone, and a gate whose padlock came apart in Haruto's hands after exactly one tug.

Now, at the end of September, the light was going gold and thin across the empty basin, and Suzu Arimura stood at the shallow end with her notebook open against her forearm, counting.

"Haruto. Miyu. Ren." Her pen tapped the page once for each name. "Nana. And me."

Five. She counted again, because five was the right number but the first count had felt too easy. Five. She counted a third time, silently, mouth moving on the names, and only then did she let her shoulders come down from around her ears.

"You know that's three times," Haruto said. He was sitting on the lip of the pool with his legs dangling over the drop, tossing a rubber ball from one hand to the other. "I've been keeping track. Three times in five minutes. At this rate you're going to wear a hole in us."

"Four is the minimum for a safety check," Suzu said, not looking up. "Three is a habit. I'm allowed a habit."

"Madam President has a habit," Haruto announced to the empty pool, in the voice of a sports commentator. His echo came back flat and small off the tiles. "Sources say it is extremely well documented."

There was a click of a pen cap, open, closed, open, closed. Suzu realized it was her own and made her thumb stop.

At the deep end, Miyu Sakane was on her knees beside a contraption that looked like three bicycle wheels, a car battery, and a music box had been forced into marriage. Copper wire ran from it down the slope of the pool floor and into a shallow, greenish puddle of rainwater that had collected over the drain. She had her sleeves rolled to the elbow and a smear of grease across one cheekbone, and she was frowning at a dial the way other people frowned at a bill.

"Resonance is stable," she called. "Mostly. There's a wobble at the low end that I don't love."

"What kind of wobble?" Suzu was already walking down the slope, notebook ready.

"The kind that means I don't know yet." Miyu wiped her hands on her trousers, which did nothing. "Honestly, it's behaving better than it has any right to. Three weeks ago it was shaking the lab windows."

"It was shaking the lab windows because you ran it off the building's own power."

"And we learned something."

"We learned that the vice principal has a very loud voice," said Ren Tachibana from the diving board.

He was seventeen, a year below Suzu, and he had positioned himself at the one spot in the pool area that let him look down on everyone else without technically being in charge of anything. His arms were folded. He had not smiled since roughly Tuesday.

"I want to say it for the record," Ren went on. "Again. This is a bad idea. We are going to pour a frequency into standing water in a place we are not allowed to be, and the best case is that nothing happens."

"The best case," Miyu said, "is that we get a clean standing wave pattern and I publish it in the club journal."

"The best case is that nothing happens," Ren repeated, "and the second best case is that someone calls a teacher."

Suzu turned a page. She had the whole thing written out already, in her small upright handwriting, with a margin for each risk and a line drawn under each answer. Water depth, under four centimeters. Voltage, under twelve. Exit routes, two. First aid kit, in her bag, checked on Friday and again that morning.

"Ren's concern is noted," she said, and she meant it, which was the part that annoyed him. "It's the right concern. Which is why I'm going to walk the whole setup one more time before we switch anything on. Miyu, show me the grounding."

"I showed you the grounding."

"Show me again."

Miyu sighed, and showed her again.

It was while Suzu was crouched beside the battery, tracing the copper wire with the end of her pen, that a small shadow fell across the page. She knew whose it was before she looked up. Nana Fujisawa was sixteen and the smallest member of the club by a head, and she had a way of arriving at the edge of your attention without ever seeming to cross the distance.

"Senpai." Nana was holding the straps of her bag with both hands. "Can I stay for this one? Please. I did the reading. I did all of it. I know what a standing wave is."

"Nana." Suzu closed the notebook on her finger. "You said your mother wanted you home by six."

"I told her seven."

"Did she say yes?"

Nana looked at her shoes. "I told her seven," she said again, which was not an answer, and which Suzu was fairly sure was the most rebellious thing Nana had ever done.

She should have said no. Every list in her notebook said no. But Nana's face had the particular brightness of someone who had been waiting a whole season to be allowed into a room, and Suzu felt the old, cold familiar thing rise up under her ribs: a sunny field, a rope bridge, a girl's voice calling her name from the wrong side of the trees.

She pushed it down. She was good at that.

"You stay with me," she said. "You don't go past the yellow line Miyu has marked. If I say step back, you step back, no questions. And if the wobble gets worse, you walk to the exit, not run, and you wait for us at the gate."

"Yes, Senpai."

"Say it back to me."

Nana, to her credit, said it back to her, all three points, in order, and Suzu wrote a small tick beside her name.

By the time they were ready the sun had slid behind the school gymnasium, and the pool had turned the blue-gray of a held breath. The cicadas had gone quiet. Somewhere past the fence a bicycle bell rang and was gone.

Miyu crouched at the controls. Haruto, who had been told to stand still and had not managed it, was bouncing on his heels with the ball held to his chest. Ren had come down from the diving board at last, and stood a careful three paces back from the puddle, his jaw tight.

"On your count, Madam President," Miyu said.

Suzu looked at them. Five. One more time. She did not say it out loud.

"Switch it on," she said.

The contraption hummed. It was a thin sound at first, almost nothing, like a wet finger run around the rim of a glass. Then the puddle shivered. Concentric rings crept out from the copper wire, regular, orderly, exactly the pattern Miyu had drawn on the whiteboard a dozen times. Haruto let out a low whistle.

"Standing wave," Miyu breathed. "There it is. Look at that. Look at the nodes."

"Recording," Suzu said, and wrote the time. 6:14 p.m. Her pen was moving faster than her thoughts. "Amplitude nominal. Ren, anything?"

"Nothing," said Ren, and there was something almost disappointed in it. "It's a puddle making circles."

Then the circles stopped moving outward.

They turned around and began to move in.

It was such a small wrongness, at first, that Suzu thought she had made an error in her own head. Ripples came from the edges toward the center, steady and slow, as if the puddle were being slowly drawn through a straw. The hum dropped a half-tone. The wobble Miyu didn't love swelled, and underneath it came a second sound, deeper than the first, a long bell-note that seemed to come up through the soles of their shoes.

"That's not mine," Miyu said. Her voice had gone very quiet. "Suzu, that's not coming from the machine."

"Shut it off."

"I'm trying. It's not" Miyu's hand slapped the switch, slapped it again. "It's not responding."

The puddle was no longer a puddle. It had no edges. Suzu looked at it and her eyes slid off the shape of it, like looking at the bottom of a pool in summer, where the depth makes a liar of the light. It was a hand's breadth wide and it was a lake. It was a hand's breadth deep and the tiles beneath it were gone, and what lay in their place was dark and clear and falling away forever, a cold blue throat with pale sunlight turning slowly at the bottom.

"Everyone back," Suzu said. "Back, back, exit, walk, don't run."

"Which exit?" Haruto said, and his voice had lost every ounce of its joke. "Suzu. The fence is gone."

It was. Where the chain-link had stood, there was only gray evening and a wall of slowly rising water, and the water was not wet. It stood in the air like glass and it was coming toward them in a long, level, silent sheet.

The floor tipped.

It did not break; it simply stopped being solid, the way a dream stops being a dream. Suzu felt the tiles go soft under her shoes and the weight of her own body arrive all at once, and then the whole basin gave way beneath all five of them, and they dropped.

She was already reaching. That was the thing she would remember afterward, the thing she would turn over in her mind on a hundred sleepless nights: she did not scream and she did not close her eyes. Her hands went out and found cloth, a sleeve, a wrist. Haruto. Miyu's collar. Ren's shoulder, which he tried to shake off and then, abruptly, did not. The cold took them like a fist closing.

Sound went out. The world became a long blue roar, and bubbles tore upward past her face, silver and furious. She kicked and could not tell which way was up. Her lungs were tightening. Her fingers were closing and opening on nothing, on water, on the sleeve of someone who had slipped away, and she counted, she could not stop counting, three, four, where is five, where is five

Then she saw the notebook.

It had left her hand. Its pages had come loose from the binding and were drifting around her, white and rippling, and for one dazed moment they looked like a shoal of small silver fish. Every list she had ever made. Every risk matrix. Every careful tick beside every name. They circled her, bright and weightless, and some part of her mind that was still counting said: you can catch them. You can still catch them. They're all you have.

A small hand closed around her sleeve.

Nana's face hung beside hers in the blue, hair floating up like smoke, eyes huge and terrified and fixed on Suzu as if Suzu were the only solid thing left in the world.

Suzu let go of the notebook.

It was the easiest decision she had ever made, and it frightened her more than the water. She opened her hand, let the pages go, and wrapped both arms around Nana, and the loose white leaves spun away into the dark like startled birds. She kicked toward the light. She did not know if it was up. She kicked anyway.

The water thinned. The roar sharpened into a ringing, and the ringing broke, and then Suzu's head cleared the surface and she hauled in a breath so large it hurt.

Sky.

Not the gray September sky over Hinode Ridge. This sky was wide and pale as the inside of a shell, laced with high thin cloud that glowed rose along its edges, and under it, as far as she could see, lay a flat bright sea. A tower rose out of the water a few hundred meters off, a long pale thing of stone and arched windows, drowned to its knees, its upper floors lit gold by a sun that sat in the wrong part of the sky.

"Nana," she gasped. "Nana, breathe. Are you breathing? Look at me."

"I'm," Nana coughed. "I'm okay. I'm okay, Senpai."

"Haruto!"

"Here!" A wet, crackling shout from her left. He was floundering toward her, hair plastered flat across his forehead, ball still tucked under one arm like a wounded pet. "I'm here, I'm here, that was the worst thing that has ever happened to me. Is everyone"

"Miyu."

"Present," said Miyu, from behind him, spitting water. She was dragging Ren by the collar, and Ren was furious and coughing and alive, which in Suzu's experience were the same thing.

Suzu turned in the water, slowly, and counted them. Haruto. Miyu. Ren. Nana, against her shoulder. Herself.

Five.

The breath she let out shook. Her eyes were stinging, and she told herself it was the salt.

"Everyone stay together," she said, and her voice came out almost steady. "Don't swim apart. We go toward the tower. It's the only thing here that isn't water."

"Is it a tower?" Haruto said. "Or is it a hallucination in the shape of a tower?"

"I don't care which, as long as it floats."

They swam. It took a long time. The sea was warm in a way that felt wrong, warm as breath, and strangely clear, so that when Suzu looked down she could see the bottom twenty meters below, a drowned street, a row of roofs, a bell lying on its side in silt. Her arms burned. Nana clung to her without complaint. And the tower, patient and indifferent, drew closer.

It was Ren who saw the stairs first. "There," he said, tight and breathless. "A landing. Stone steps. They go down into the water."

They were broad, pale, worn into shallow dips by countless feet. Suzu reached the lowest step and gripped it with her free hand and felt, for the first time since the floor had vanished, something solid under her palm. She got Nana onto the stone, then Miyu, then she pushed Haruto up from behind while he said, "Thank you, thank you, I love you all," and then she was dragging herself out, her wet skirt a lead weight, her legs refusing to hold.

She made it up three steps and her knees buckled. She sat down hard on warm wet stone.

"Okay," she whispered, to no one. "Okay. Okay. We're okay."

A shadow fell across her.

She had not heard anyone come. One moment the landing above her was empty, and the next a figure stood three steps up with the low light behind him, tall and straight, in a pale high-collared coat edged with navy that fell nearly to his knees. His sleeves were rolled once, neatly, to the forearm. His skin was a soft pearly gray, like the inside of a mussel shell, and it caught the light in a way no human skin ever had. His hair was short and ash-dark and damp at the temples. In one hand he held a thin, flat ledger bound in something pale and waxed, and in the other a slender stylus.

He looked down at her, and she looked up at him, and for a moment neither of them did anything at all.

His eyes were a violet gray, cool and exact. As she watched, a thin, translucent membrane slid sideways across them, quick as a blink, a veil of clear second lid like a creature of the deep sea. It withdrew. He did not seem to notice he had done it.

"Don't move," he said.

Suzu, who had been about to move, did not move.

His voice was even, low, and completely without alarm. It was the voice of someone reading a figure from a column. He crouched on the step above her and set the stylus against the ledger, and his gaze went past her, over her shoulder, to the four wet shapes huddled behind her on the stone.

"Five," he said. "Arrived through the old gate at the sixth bell. All alive, no apparent injury." The stylus scratched once. Then his eyes came back to Suzu, and there was something in them she could not read: not warmth, not suspicion, but a kind of intense, careful attention, as if she were a sentence in a language he had only ever seen written down.

She tried to stand. Her legs gave out again, and she pitched forward, and his hand came out and closed around her wrist.

His fingers were cool and dry, impossibly dry, though he had to have crossed wet stone to reach her. His grip was not rough. It simply did not let go. It held her exactly where she was, as steadily as a rail, and the contact ran up her arm like a struck note.

"I said," he repeated, very quietly, "don't move."

Then he drew her up the last step as though she weighed nothing, and set her on her feet on dry stone, and did not release her until he was sure she would not fall.

"You are being registered," said the stranger, and across his pearl-gray face, for one fleeting instant, something very much like curiosity passed. "Please stay still. It will be easier for everyone."

Suzu stood there, dripping, with her heart stuttering against her ribs, and found that she could not remember a single item from any list she had ever made.