At first it was just a faint tang in the back of the throat or the feathery farthest-out edges of the gills. The coastal patrols came back itchy and out of sorts, and everyone who ventured out there complained of the taste: bitter and sour at once, like something growing had twisted, or died.

Murex mostly stayed inside the undersea city. Of course he needed to use his gills from time to time, as anyone did if they didn’t want their gills to get stiff and uncomfortable. But chemists were too valuable to send out on patrols, which were a job for the restless youth who hadn’t yet found their current in life. Most of his swims were in the gentle, curated coral gardens that were the wet part of the city, just outside the airlocks of the dry sectors.

Besides, his boyfriend Zoanthus was a rare surface person dwelling in the undersea city, so they couldn’t do swimming dates like normal couples.

But his sister Teal was, like many doctors, implacable. When she checked her fourth patroller into the infirmary with a wandering mind and reddened, itchy gills, she insisted that Murex accompany her out to bring back a water sample—in protective suits—and Murex felt tidally pulled along, as he so often did when Teal took a notion.

Two of the patrollers they brought with them refused to wear the suits. Teal agreed reluctantly that they would be able to sense the greatest concentration of the contamination in the water that way. But as they approached the coast, Murex could feel the unusual warmth in the current even without the patrollers’ signal. Soon an orange tinge threaded through the water itself. Teal waved the unsuited patrollers off home: they were not needed to spot the surrounding contamination.

He felt squirmy and relieved to return to the clear waters around the city, even more so to be in the airlock and using his lungs again. He stared at the sample vials in sick fascination. His gills itched just looking at them. The contamination that had come upon them gradually now looked fungal, silty, chokingly intense.

But it took much longer to identify in his lab than he’d expected—much longer than Teal found satisfactory. She took to hanging around glaring at him, as though his work would go faster with her sloshing the tank. The orange substance was complicated and organic, and he began to wonder if they had an algal bloom or something worse.

Teal, grousing, put out word that the patrollers should definitely not go out without suits until further notice. By then everyone knew someone who was still in the infirmary, immersed in constantly cleaned water, so complaints were muted.

The city council, previously ignoring any trivial details about the waters of its outskirts, began to lean on Murex’s door, pressing for answers he couldn’t give. When he finally did figure out a long organometallic formula, it had very strange uses of benzene rings. There was no way it was natural.

Frowning, he brought a sample home and set it on the dinner table. He saw Zoanthus’s gaze leap away from it but said nothing at first, putting bowls of fresh kelp and sea cucumber on the table for them to share. Zoanthus shoveled food in silently, completely unlike his usual chatty cheer.

“So I figured out a formula for this orange stuff,” Murex finally said, “but that barely tells me more than I knew before. Except it’s definitely artificial. Do you know what it is from your time on the surface?”

Zoanthus's face shut down like gills in fresh air. “No.”

“Are you sure? You could just—”

“I’m not a scientist like you, Mur, I don’t know what the hell it is. I just grow plants. I deal with green, growing things. It’s not my job.”

“Okay, sure,” said Murex, bewildered.

“I don’t think I’m hungry any more.”

“Okay,” Murex repeated. He picked at his own dinner while Zoanthus dumped his bowl in the sink, shuffled off into the bedroom, and shut the door.

They’d had surface people in the undersea city before. Most of them didn’t stay long, wilting in the enclosed space inside the airlock and unable to fully immerse in the beauty of the octopus gardens and fish schools outside.

Zoanthus was different. He had been there for five years, long enough to have friends and work and ideas about how to prepare deep-sea kelps for parties, enough that he could speak up in city meetings without the neighbors rolling their eyes about his ignorance of sea customs and needs. Enough that he and Murex had moved in together.

Which did not mean that Murex understood him, as moments like this one underscored all too well.

Murex cleaned up the dishes—something they would usually do together—and hesitated at the door to their bedroom. Zoanthus had these moods. He needed his solitude. What he needed, Murex suspected, was a horizon to gaze at, a far-off vista, somewhere to let the wind ruffle his straight black hair.

He was the only person in the city who had hair.

The sea, while vaster than humans of any subspecies could comprehend, closed around them rather than opening.

No one had died of the contamination, yet. And Murex could not let the strange dictates of his boyfriend’s land soul, appealing though their mystery could be, endanger them.

He tapped on his own bedroom door and went in without waiting for an answer. Zoanthus stared at him from the bed, dark eyes red-rimmed and miserable.

“Tell me,” said Murex gently. “I don’t have to tell anyone else. But tell me.”

“When I was a soldier,” said Zoanthus, and Murex sighed like the wind calming down after a typhoon. Zoanthus did not seem to be able to go on. Murex crossed to sit next to him and took his hand. He waited.

“When I was a soldier,” Zoanthus repeated, “there was a—a drug. They gave us. In the war.”

“I see,” said Murex, and waited again.

“Your orange … stuff. It’s related to that. It’s a … a byproduct. Not useful for our dreaming, not useful to keep us alert or any of the other…key things they wanted from us. But part of the process. The factory, the factory atop the cliffs … .”

Murex tightened his hand, long fingers flexing around Zoanthus’s stubby ones.

“I came here to get away from all that,” Zoanthus whispered.

Murex lifted their twined hands to his mouth. Zoanthus closed his eyes. “Don’t worry,” Murex murmured. “We’ll handle it, I promise.”

Zoanthus relaxed into him, all too eager to focus on something else. His dry, firm surface person mouth was irresistible, and while Murex remained baffled, even worried, at the way the conversation had gone, he was able to let it slide downstream for the moment, to immerse himself in his love’s body.

They were drifting off after, mutually washing away into sleep, when Zoanthus whispered again, “I just grow plants, I don’t know the science,” and Murex was jolted out of his doze.

He lay awake for hours, wondering why he had believed him so much more readily when he’d only said it once.

Zoanthus got up before Murex and made breakfast for them both, which was rare. Rarer still, he was out the door with a quick kiss and a mouthful of kelp excusing him from explanations. Murex frowned after him, but had too much to do to pursue the matter. He found Teal in the outer rooms of the infirmary, crease-browed and surrounded by paperwork. He told her what Zoanthus had said about the substance—and what he’d reiterated.

The lines of her scowl replicated his own. “What’s going on there?” she demanded.

“I don’t know,” he admitted, “but I’m going to find out.”

“So we’re looking for industrial runoff, perhaps a pipe with emissions under the water line, or perhaps along the shore?”

“I suppose,” said Murex. He fluttered his hands and his gills in frustrated fidgets. “Even if we do know where it’s coming from—he seemed so upset, Tee. I don’t know what we’re going to do about it, if it’s dangerous enough to scare him that much. I don’t know how we’re going to convince him he’s safe.”

Teal sighed. “We’ve always found a way, with Zo. I’m more concerned about him actually being safe—along with the rest of us. None of my patients has even gotten to go home to convalesce yet.”

Murex grimaced. “I’ll do what I can to figure out some kind of binding agent. It’s not going to be easy.” She nodded absently, her mind already back to her patients.

Zoanthus had not had a name when he arrived below the waves—or rather, he wouldn’t tell it to anyone. “Give me a new name,” he repeated, until Teal had pity on him and chose a kind of brightly colored coral that her aunt had nurtured in the city’s gardens. And so Zoanthus the gardener was born, earnest, cautious, dedicated to his boyfriend and his boyfriend’s sister, to the children of their block and the plants of the airy green rooms.

Murex had wondered, and eventually had asked: won’t they follow you here? Won’t they find you? But Zoanthus was confident on that point, not tense: when the armies learned their amphibious human experiments were not reliably a naval asset, the experiments themselves were not precisely forgotten, but no longer regarded. No one would think to look for him under the waves—they would presume nothing that might be called human lived there. Zoanthus himself had only found them by whispered rumor and accident.

Whatever the soldier’s name had been, Zoanthus wanted to bury him at sea. And he had succeeded, until now.

Murex turned back to his lab bench. He had always been so proud of the way his city gave a refuge to those in need, particularly his partner. Now he would have to be the front lines of that, the protector of the city with his vials and his experiments.

It was more tedious than he expected to be the hero. He knew that the waste molecule was just an organometallic snippet of junk, but it was hard not to personify the wretched thing when it kept wiggling out of bonds that would neutralize it. One of Teal’s patients recovered, albeit with badly damaged gills. Two died.

Murex barely saw Zoanthus for more than a quick breakfast here, a kiss on the cheek there, and back to work, back to the lab, collapsing into sleep and starting over again the next morning. He wasn’t even sure what Zoanthus was doing with the hours they usually spent together. He wasn’t cooking elaborate meals or bringing complicated arrangements of flowers or kelp into their home. He didn’t seem to be out with friends.

From the funky surface smell of the apartment, it was possible that he was just lying in bed staring at the wall. But Murex didn’t want to think about that. He didn’t have the time to think about that.

Finally, with the help of a few others Teal had commandeered for his project, Murex managed to kludge together a solution. It would take vigilance and renewal, but they could apply a permeable coating to the cliff that wouldn’t let the toxic waste particles pass through, while ordinary groundwater would. He returned home for supper that night in triumph, expecting Zoanthus to be relieved to be able to return to ignoring the surface. But instead he saw more fear in his love’s wide eyes, smelled it in the unnatural dryness of his breath.

“It’ll seal it in?” Zoanthus squeaked, back to the panic Murex had not expected to see again.

“It’s their waste, it’s theirs to clean,” said Murex. “Bad enough that it’ll still be in the rock and soil—the little microorganisms didn’t make this foul stuff—feh, I feel like I’ll have to swim for a week before my fingers feel really clean—and yes, before you ask, I always used gloves to handle it. Teal drilled that into my head well enough.”

But Zoanthus wasn’t listening. “Then they’ll have it … themselves, backed up into the land.”

“Yes.”

“But they—but it—”

Murex took his hands. “Love, we’ve lost people to this. Lost them. And I’m not going to lose more. I’m not going to lose you.” He was surprised to see Zoanthus was crying. Surface people’s bodies handled moisture so differently that he found it hard to predict even now.

“Look, I know it’s … it’s not good,” said Zoanthus, not meeting Murex’s eyes.

Murex waited patiently, just looking at him, until Zoanthus finally looked up under his long, wet lashes. “I know it’s bad,” he adjusted. “But if you—if you—”

“It is bad,” Murex confirmed. “Yes. It’s poisonous. Your people don’t live in the sea, but they live by it, they fish from it, how do they not know that this is hurting them too? How do they not—”

“They are so constantly hurt,” Zoanthus whispered. “Everything hurts, on land, everything, always.”

“You came here to get away from that,” said Murex softly.

Zoanthus nodded vigorously, his face melting into relief that Murex understood. Murex flinched to have to go on, but he knew he did.

“You came here to get away from that, and it won’t help anything if we let it reach you here.”

Zoanthus’s mouth opened and closed. “But—but—”

“I can keep you safe,” Murex said. “I can’t protect your people from themselves.”

Zoanthus made a sound so cracked and broken that Murex was not sure any of his own people could replicate it, despite their common genetic roots. He reached for his love, but Zoanthus flailed and knocked his hands away. Murex stood staring helplessly for as long as he could bear, then retreated to his lab.

Teal was there half an hour later. “I know where Zoanthus has been going.”

Murex blinked at her from his lab bench. “You what?”

“He’s been suiting up and going outside. He’s out there now.”

“Where?” he demanded. Teal shrugged. Murex slammed his hand down on the lab bench, long fingers vibrating with frustration. The sea was too wide for him to be able to find Zoanthus on a random wander, and who could tell where he’d have chosen to go?

Murex had a guess, though. He slammed past Teal, who grabbed him by the shoulder. “Mur. Don’t—don’t do anything rash.”

“I’ll wear a suit,” he snapped.

“I meant—be kind to Zo. Be careful with him.”

He stared wearily at her. “I’m more careful with him than he is with himself.”

Teal tried to smile. “Well. Keep it that way.”

He swam in the strange clamminess that was a suit instead of his skin. It was always scary and uncomfortable to breathe through his lungs instead of his gills when he was in the water, and his claustrophobic sense of wrongness was worse for knowing that the water was unsafe if the suit did fail.

He swam up and down under the cliffs, looking for Zoanthus in the orange murk. It had been a long day—a long month—and he grew more frantic, more exhausted, with each passing current. Finally, through the burnt-looking haze of water, he saw a flash of matte grey that was not a fish. He put the last of his energy into a burst of speed and caught Zoanthus by the foot.

He realized, belatedly, that Zo’s head had been above the waves. But it was too late, he pulled him down under again, thrashing frantically. Zoanthus kicked and lashed out. Murex let go, tried to push himself away in the water far enough that Zo could see it was him.

Zoanthus’s body stilled to a gentle treading motion. Murex motioned to him curtly, trying to take Teal’s advice in mind. Zo was a slower swimmer even after this many years, even with Murex in a suit. It took them long enough to get home that Murex was completely wrung out. He didn’t even know what he would say to his love when they were inside the airlock. But there Zo stood, balanced against the changing bench they had there for suit-wearers, backlit and haggard.

“What were you doing out there? What were you thinking?” Murex had expected it to come out as a snarl, but he was so exhausted he mumbled instead.

“I was a soldier,” Zoanthus said, as though it answered anything. “I wasn’t lying to you about that.”

“You weren’t—Zo? What?”

“But they trained me to be a field medic, and I worked my way up from there.”

Murex tensed. “Up to what?”

“I still want to be a gardener.”

“Zoanthus. You’re scaring me.”

Zoanthus held out his hand. “Let’s go to your lab. I’ve been looking and thinking and trying and…let me show you how to make it self-catalyzing.”

“Zo,” Murex breathed.

“I wanted to just neutralize it. Maybe with a plant, I thought maybe some bioremediation could ... you’re good at that here, though. You do a lot with that. And nothing I came up with worked to make the toxins disperse gently. I’ve been trying and trying to figure out how.”

“No wonder you’re as tired as I am,” Murex whispered.

“So if I can’t neutralize it, I have to contain it. I know that. This is my home too. And ... they could be safe any time, if they just stop making the drug to force down the soldiers’ throats.” He rubbed the palms of his hands convulsively. “They could. They could stop any time.”

“They could,” Murex agreed, but he knew what Zoanthus was really saying: that they wouldn’t. And that he was going to be an instrument of consequences for his own people.

For his own former people. Murex put an arm around his lover’s waist, leaned into him, and staggered off to the lab so that they could improve his formulas, together.

It took several more days to make the compound, once they’d worked it through together. Murex could have sent someone else out to deploy the solution. But he felt right doing it himself—swimming in the quiet dark, even with the unaccustomed suit between himself and the water. It felt final, but he knew that there would be wave after wave of consequence. He hoped they were both prepared.

Murex looked up at the cliffs. They were studded with lights, yellow-orange and steady in the night. They only felt as distant as the stars; their proximity was so close he might actually touch them. The people who kept them lit, to and fro, shared the ocean with him, and the sky. They had given him Zoanthus—his partner in more ways than ever, now.

He didn’t want to hurt them. But he couldn’t let them spread their sickness.

He opened the vial of sealant seeders to the water and turned to swim home, to Zoanthus and the city they shared.