They left with the tide.
Dawn was a thin line of copper on the eastern water and the harbor was full of people who had come to watch something they didn't believe. Twelve ships in a staggered line, sails catching the offshore breeze, hulls riding low with provisions and the particular weight of purpose. Moss stood at the helm of the Blacktide and watched the city shrink.
Tidemouth looked small from the water. It always did. The breakwater curved out like a crooked arm and the buildings behind it were grey and brown and clustered together the way cold animals cluster. Smoke from morning fires. The observatory on the ridge, a dark tooth against the sky. And on the breakwater itself, figures — dozens of them, standing in the early light, watching the fleet go.
He found Kael without trying. She was at the end of the breakwater, standing apart from the others, her arms at her sides. She didn't wave. She watched. The distance between them opened like a wound that had been waiting to tear, and Moss looked at her and she looked at him and neither of them moved.
He turned south.
Captain Lin was at the stern rail of the Blacktide, talking with Davi the quartermaster about water stores. Lin was sixty-three and had the body of a rope — thin, hard, impossible to break. She'd chosen her crew the way she chose fish: by looking them in the eye and deciding if they were worth the trouble. She'd chosen Moss first. He hadn't asked why. He knew why. He was the best navigator in Tidemouth and the only one who'd been south twice and come back both times, which was a record that said less about skill than about luck, but luck was a currency Lin respected.
The fleet spread wide as they cleared the harbor approaches. Twelve ships, twelve courses. The plan was a fan — each vessel taking a slightly different heading, the outermost ships thirty degrees apart, so that the fleet covered a corridor nearly two hundred kilometers wide. If one route was passable, at least one ship might find it. If no route was passable, the width wouldn't matter.
Moss had the charts. They were mostly blank. South of the coastal shelf, the old maps ended and the new ones hadn't been drawn because nobody had gone to draw them. He had depth soundings from Before — unreliable after five centuries of seabed change — and current estimates from the scholarly council that were educated guesses dressed in numbers. He had the stars. He had dead reckoning. He had his hands and his eyes and the water itself, which told you things if you knew how to listen.
The first week was familiar ocean. Dangerous, but the kind of dangerous Moss understood — cross-currents at the shelf edge, sudden fog banks, the occasional whiff of sulfur from a coastal dead zone that the wind pushed offshore. The crew settled into watch rotations. The Blacktide cut south at eight knots with a favorable wind and the sea was grey-green and alive with whitecaps and Moss navigated and did not think about Kael standing on the breakwater and did not think about his dream and did not think about the fact that twelve ships was not very many ships for fourteen thousand kilometers of water that wanted to kill them.
He thought about it anyway. Every night, in his hammock, with the hull creaking around him and the deep Atlantic rolling beneath. Twelve ships. A hundred and eighty souls. Against an ocean.
The odds were bad. He'd done the arithmetic himself, sitting in the harbor office with Lin two months ago, scratching numbers on scrap paper. If each ship had a one-in-three chance of surviving the crossing — which was generous — then the probability of at least one reaching the other side was better than ninety percent. But one-in-three was generous. One-in-five was more honest. One-in-five put the fleet survival odds around ninety-three percent for at least one ship. Which sounded good until you counted the dead.
At one-in-five, nine or ten of the twelve would be lost. A hundred and thirty-five people. Give or take.
Moss had signed on anyway. So had everyone else. That fact sat in his chest like ballast — heavy, low, stabilizing.
Day twelve.
The morning was flat. No wind. The sea was a plate of dark glass and the sky was a pale nothing and the sails hung limp and the fleet drifted on current alone. The kind of morning that in northern waters meant a lazy day of mending and waiting. Down here it meant something else.
Moss smelled it before the signal came. A faint sourness in the air, like eggs left in the sun. His body knew what it was before his mind caught up — his hands were already moving, pulling the cloth mask from his belt, pressing it to his face.
"Masks on," he said. Not shouted. Said, because panic traveled faster than poison and was harder to survive.
The flag signal came from the Resolute, the lead ship two kilometers ahead. Yellow over red. Sulfur detected. And then, before the flags had finished their climb, a second signal — black over black. Crew down.
The Resolute was close enough to see through a good glass. Moss held the telescope steady and watched three figures on the foredeck crumple. Not dramatically. Not clutching their throats or screaming. They just folded. One was walking and then wasn't. One was sitting at a rail and slumped sideways. The third made it two steps toward the hatch before his legs quit.
Three hundred parts per million did that. Maybe more. At those concentrations, the gas paralyzed the respiratory center before the lungs could react. You breathed it in and your brain forgot how to tell your body to breathe out. The old texts called it the "knockdown effect." Moss called it what it was: the sea killing you so fast you didn't know you were dead.
The Resolute was turning. He could see the rudder bite, the hull beginning to swing. But the air was still. No wind to push the gas, which meant it pooled on the surface like an invisible lake, and the Resolute was sailing through it on momentum alone, turning slowly, too slowly —
Two more fell. One on the quarterdeck. One in the rigging, who dropped six meters to the deck and didn't move.
"All ships come about," Lin said. Her voice was flat and dry. She'd already done the math. "Signal the fleet. Southeast heading, all speed."
"There's no wind," Davi said.
"Then sweep. Get the boats out. Row if you have to."
They rowed. The Blacktide had four longboats and they put every spare hand on the oars and dragged the ship southeast while the H₂S concentration crept upward and the cloth masks, soaked in bicarbonate solution, did what they could, which was not much. Moss tasted copper. His eyes burned. Around him, crew members coughed and spat and pulled at the oars with the grim efficiency of people who understood that stopping meant dying.
It took forty minutes to clear the zone.
The Resolute cleared it too, eventually. They lost five. Three on the foredeck who never got up, and two who collapsed during the turn. The bodies were wrapped in canvas and kept, because burial at sea was a northern tradition and down here the water would dissolve a body faster than grief could process the loss.
Moss stood at the rail afterward and looked at the water. It was calm and dark and there was nothing to see — no color change, no bacterial mat, nothing to mark where five people had just died. The H₂S had vented from below, a bubble of ancient death rising from an anoxic layer, and the surface had given no sign. No warning. No purple-green scum. Just calm water on a windless morning and a smell that was already gone.
That was the first lesson of the Crossing. The ocean didn't negotiate. It didn't threaten. It killed you on a clear morning in calm seas and then it looked exactly the same afterward.
He noticed it on day fifteen.
The signal from the south — the one Kael had first detected, the one that had started everything — had changed. Moss wasn't the signal operator. That was Femi, a young technician the scholarly council had placed on each ship to maintain the VLF receiver. But Moss checked the transcripts every morning because the signal was the only information source they had, and information was navigation, and navigation was his job.
The mathematical exchanges were still there. The structured back-and-forth that Kael had described — arithmetic, then geometry, then physics constants. But threaded through the latest transmissions were sequences that didn't fit the pattern. Clusters of numbers that repeated at regular intervals, always in groups of three.
Moss stared at them for an hour. He wrote them out on the chart table, grouping them, looking for structure. He was not educated — he'd never attended the scholarly sessions, couldn't read the old languages, didn't know the mathematics beyond what navigation required. But he knew numbers that meant positions. He'd spent his life converting numbers to places on water.
Three values. Repeating groups. The first value in each group ranged between negative sixty and negative five. The second between negative forty and ten. The third was always a date offset — days from now, counting forward.
Coordinates. And times.
He plotted them on the chart. The points formed a path — not their path, but a negative space. The coordinates marked locations along their probable route, each tagged with a date. And when he overlaid the fleet's projected headings, the marked locations sat directly on the routes of four of the twelve ships.
Someone was telling them where not to go. And when.
Moss went to Lin. He showed her the chart. He didn't explain the theory. He pointed to the numbers and then pointed to the positions and said, "These are warnings. Someone south of us knows where the kill zones are and they're telling us."
Lin looked at the chart for a long time. She was a woman who trusted the sea the way she trusted a knife — useful, necessary, would cut you the moment you forgot what it was. She didn't trust things she couldn't verify.
"How?" she said.
"I don't know."
"Who?"
"I don't know."
"And you want to change our heading based on numbers from someone you can't identify, transmitted by a method you don't understand, telling us to avoid locations you can't verify are dangerous."
"Yes."
Lin tapped the chart. Her nail was cracked and yellow and hard as shell. "What's your gut say?"
"My gut says whoever built a language out of mathematics and taught it to Kael across fourteen thousand kilometers of ocean didn't do it to sink twelve ships."
Lin adjusted course. She signaled the fleet with the new waypoints. Most ships acknowledged. Two didn't — the Cormorant and the Kingfisher, running the westernmost routes, captained by men who trusted their own judgment over numbers from a stranger.
The storm hit on day seventeen. A corridor of black cloud and vertical rain and waves that built from three meters to fifteen in under an hour. The Blacktide rode it out, heeled over and taking water, because the waypoints had routed them east of the worst of it. They caught the edge. The edge was enough — twelve hours of holding on, of pumping bilges, of watching the horizon tilt and heave. Moss lashed himself to the helm and steered by feel because there was nothing to see.
The Cormorant and the Kingfisher were in the corridor's center. The fleet never heard from them again.
Ten ships. Day seventeen.
The Grey Heron went on day twenty-one. Structural failure. Her hull had been exposed to sulfidic water during the first kill belt, and the corrosion had been working since — steel dissolving at millimeters per week, the plating thinning from the inside where you couldn't see it. She reported taking water in the forward hold at dawn. By noon her bow was low. By evening the signal operator transmitted a final message — "pumps failing, abandoning" — and then silence. The sea was too rough for the nearest ships to attempt rescue. The Grey Heron went down with twenty crew, and the ocean closed over her like a mouth.
Moss marked her last known position on the chart. He marked the Cormorant and the Kingfisher too — approximate positions, because he didn't know exactly where they'd been when the storm took them. Five ships lost. Seventy-odd people. He wrote the numbers in the margin of the chart in small, careful figures, because if someone someday found these charts, they should know what happened here.
He did not grieve. Not yet. Grief was a current that would pull you under if you entered it while you were still swimming. He would grieve when he reached land or when the sea took him too, whichever came first.
Day twenty-three. The Basalt reported a crew illness — vomiting, confusion, bleeding from the gums. Sulfide poisoning, chronic and low-level, from a slow leak in the hull that had let anoxic seawater into the bilge. They sealed the leak. Three crew died anyway, over two days. The Basalt kept sailing.
Day twenty-five. The Northmark stopped answering signals. No distress call. No flag communication. The ship behind her — the Tidefall — reported seeing the Northmark on the horizon at dawn, sailing normally, and then not seeing her at dusk. Gone. No wreckage. No bodies. The ocean had swallowed her whole and left no explanation.
Moss stood on the deck that night and looked south. The stars were different here — constellations shifted, the pole star lower, unfamiliar patterns emerging from the southern sky. He was farther south than any living person he knew of had been. The water beneath him was three thousand meters deep and black as tar and the air smelled clean, which meant nothing, because it had smelled clean on day twelve too.
Six ships. Day twenty-five.
They sailed on. The waypoints kept coming — coordinates and dates, threading through the mathematical exchanges like a second conversation happening inside the first. Moss plotted each one. He adjusted course. The waypoints routed them through corridors that seemed arbitrary until you checked back three days later and heard from another ship that the water in the avoided zone had been purple-green with bacterial mats, or that the barometric pressure had dropped to storm levels, or that the air had tasted of sulfur.
Whoever was sending the data knew this ocean. Not in the way Moss knew the northern waters — by experience, by feel, by generations of accumulated knowledge. They knew it the way you'd know it if you could see it from above. As a system. As a pattern of moving kill zones and storm corridors and safe passages that shifted with the seasons and the currents and the deep chemistry of an ocean that had been sick for five hundred years.
Moss trusted the waypoints. He couldn't have said why, except that they worked. Every course correction they suggested had been right. Every warned location had proven deadly. And the alternative to trusting them was navigating blind through an ocean that had already taken half the fleet, which was not an alternative at all.
Day thirty.
Six ships remained. The Blacktide. The Resolute (scarred, short-crewed, still sailing). The Basalt. The Tidefall. The Windglass. The South Wind. A hundred and six people from a hundred and eighty. The rest were in the water behind them — wrapped in canvas or trapped in hulls or simply gone, taken by a sea that had no malice and no mercy and no particular interest in whether they lived or died.
Lin came to the helm at sunset. She stood beside Moss and looked south, the same way he did, the same way they all did now — south, always south, because there was nothing behind them worth turning back for and everything ahead was unknown.
"How far?" she said.
"Nine thousand kilometers. Maybe more. Six weeks at this pace, if the wind holds."
"It won't."
"No."
She was quiet for a while. The ship rose and fell on long swells that had traveled from somewhere Moss couldn't imagine. The water was dark blue, almost purple, deep and cold.
"The waypoints," Lin said. "You trust them."
"They've kept us alive."
"They've kept us alive so far."
"That's the only kind of alive there is."
Lin almost smiled. It was a small thing, a tightening at the corners of her mouth that someone who didn't know her would have missed. She gripped the rail and looked at the horizon and said nothing else, and Moss steered by the stars and by the numbers from the south and by the stubborn, irrational conviction that the white shore in his dreams was real and that someone was waiting there and that the ocean, for all its killing, would let him through.
Six ships of twelve. Three weeks out. Nine thousand kilometers to go.
He held the course.