The dive clock on Kael-7's left forearm read 04:17:33 when the signal first appeared—a flicker on the sonar display, a ghost in the deep-range band, a frequency that should not have been there because nothing in the Flood had transmitted on that band for forty-seven years. Kael hung suspended at twenty-eight meters, the water pressing against the gill slits along her ribs with the familiar pressure of a second heartbeat, and watched the signal trace across the display like a finger drawing a line through dust. The frequency was 432 hertz. She knew this because her auditory implant had identified it automatically, the data scrolling across her retinal overlay in pale green text, and she knew what 432 hertz meant because she had been a music student before the Flood, before the water rose past the fourth floor of every building in London, before the corporations built their arcologies above the new sea level and left everyone else to drown or adapt or die trying. 432 hertz was the frequency of A above middle C in the Verdi tuning system, the system that opera singers had preferred for centuries because it was said to resonate with the natural vibration of the human body. Kael-7 had not heard a human voice singing in Verdi tuning for forty-seven years, because she had been submerged for forty-seven years, and the only voices she heard now were the voices on the corporate broadcast channels and the voices of the other scavengers on the salvage nets and the voice of her own mind counting down the mutations.

She was on her seventh body. That was what the 7 meant—not a designation, not a serial number, but a count. Seven adaptations since the Flood. Seven times she had gone under the knife in the modification chambers beneath the arcologies, trading pieces of her original biology for the ability to dive deeper, stay longer, survive in water that would crush an unmodified human like a tin can in a fist. The gills had been the first modification, back when she was still Kael-1, back when the water had only reached the second floor and the corporations were still pretending that the Flood was temporary, that the seawalls would hold, that the climate models were wrong. The gills had cost her the ability to breathe air without pain—every breath above the surface now felt like inhaling sand, a constant reminder that the old world was not her world anymore, that she belonged to the water now, that the water was the only place that would have her. The second modification had been the webbing between her fingers and toes, the third the nictitating membranes that slid across her eyes when she dove past thirty meters, the fourth the sonar implant in her temporal bone, the fifth the pressure-resistant cartilage that replaced her ribs, the sixth the thermal exchange system that let her blood function at temperatures that would kill an unmodified human, and the seventh—

The seventh had been the voice. Not the loss of her voice—that would have been simpler, cleaner, a subtraction rather than an addition. The seventh modification had been a vocal synthesizer implanted in her larynx, a device that translated the frequencies she heard underwater into signals that the corporate salvage nets could process, because the corporations did not pay for music, did not pay for art, did not pay for anything that could not be quantified and catalogued and sold. The vocal synthesizer had been supposed to make her a better scavenger, a more valuable asset, a more efficient tool for the extraction of pre-Flood artifacts from the drowned buildings of London. What it had actually done was make her unable to speak above water at all, and unable to stop speaking underwater, because the synthesizer was always on, always processing, always translating the sounds around her into data streams that the corporate servers received whether she wanted them to or not. She had not sung a song—not in her own voice, not in any voice—since the seventh modification, because singing required silence, and silence was the one thing the synthesizer could not produce.

The signal on her sonar display pulsed again. 432 hertz. A above middle C. Kael adjusted her buoyancy compensator—a vest of compressed helium bladders that had replaced the scuba gear of the previous century—and angled herself downward, toward the source. The water at this depth was the color of old tea, thick with sediment and the microscopic organisms that had colonized the submerged city, and the buildings rose around her like the bones of a dead whale, their windows dark, their doors gaping, their interiors filled with the slow accumulation of forty-seven years of silt and salt and the particular silence that comes when a place has been abandoned by everything except the water itself.

She was in what had once been the financial district. The buildings here were taller than the buildings in the residential zones, and their upper floors still broke the surface, still reached into the air like the fingers of a drowning man, still displayed the faded logos of banks and insurance companies and investment firms that had moved their operations to the arcologies decades ago and left their physical premises to rot. Kael had scavenged this district before—had pulled filing cabinets and computer servers and the occasional safe from the offices on the lower floors—but she had never gone below thirty meters here, because below thirty meters the pressure began to affect the structural integrity of the buildings, and the risk of collapse was too high for the salvage the corporations were willing to pay. Below thirty meters was where things went that did not want to be found.

The signal was coming from forty-one meters. Kael checked her dive clock—04:19:02, two hours of oxygen remaining in the rebreather canisters, four hours of battery in the thermal system, six hours before the tide changed and the current through the district became too strong to navigate. She had time. She had time and she had curiosity and she had the particular recklessness that came from knowing that she was already seven mutations away from the person she had been, that the person she had been was already dead, that every dive was a negotiation with a past that no longer recognized her.

She dove.

The building at the center of the signal was a concert hall, or had been—a neoclassical structure with columns and pediments and a dome that had collapsed inward decades ago, leaving a jagged hole through which the water flowed like breath through a broken lung. Kael entered through a window on the fourth floor, pushing aside a curtain of algae that had grown so thick it felt like fabric, and found herself in what had once been a rehearsal room. The floor was carpeted in silt. The walls were lined with waterlogged sheet music, the notes blurred into illegibility, the paper dissolving into the water like sugar into tea. A piano stood in the corner, its keys stripped of ivory, its strings rusted into silence, its body swollen with the slow absorption of forty-seven years of seawater. Kael touched one of the keys. It did not move. Nothing in the Flood moved unless the water moved it.

The signal was coming from below—from the main auditorium, she guessed, the space beneath the collapsed dome. She swam through a corridor lined with dressing rooms, past mirrors that reflected nothing but the green murk of the water and the dim glow of her retinal overlay. The synthesizer in her throat was humming, processing the ambient noise of the building—the creak of settling timbers, the ping of distant metal, the constant low-frequency drone of the current moving through the corridors like blood through veins. The signal was clearer now, stronger, a pure tone that cut through the ambient noise the way a searchlight cuts through fog.

She found the source in the center of the auditorium. It was a recording device—a pre-Flood device, solid-state, military-grade, the kind of device that had been designed to survive electromagnetic pulses and nuclear blasts and the collapse of civilization. It was sitting on what had once been the conductor's podium, still sealed, still active, still transmitting on a frequency that should have been silent for forty-seven years. Kael picked it up. Her hands were webbed and clawed, the fingers longer than human fingers, the joints reinforced with carbon fiber, but they were still her hands, still capable of the gentleness required to hold something fragile, something precious, something that had been waiting for someone to find it for almost half a century.

She activated the playback. The synthesizer in her throat went silent—the first silence it had produced since the seventh modification—and a voice filled the water.

It was a woman's voice, singing in English, or perhaps in Welsh—the accent was old, pre-Flood, the kind of accent that had belonged to a world that still had borders and nations and the particular arrogance of people who believed that the land would last forever. The song was about a world on fire, or a world underwater, or perhaps both—the words were difficult to make out through the water and the static and the distance of forty-seven years, but the melody was clear, and the melody was 432 hertz, A above middle C, the frequency that resonated with the natural vibration of the human body. Kael-7, who had not heard a human voice singing in her own register since before the Flood, who had not felt her own body resonate with anything except the pressure of the water and the hum of the synthesizer and the countdown of the mutations, stood in the ruined auditorium of a concert hall beneath forty-one meters of seawater and listened.

The voice sang for three minutes and forty-seven seconds. When it stopped, the recording device emitted a single tone—a data transmission, Kael realized, encoded in the subsonic range, a set of coordinates and a timestamp and a message that her retinal overlay translated automatically: This is the eighth recording. There are twelve more. Follow the signal.

Kael checked her dive clock. 04:24:11. She had time. She had time and she had a signal and she had the first thing that had felt like a reason to surface since the seventh modification. She tucked the recording device into her salvage bag and followed the coordinates, and she did not think about what the eighth recording meant, did not think about why there were twelve more, did not think about what she would be when she had followed the signal to its end and counted down to zero.

The second signal was at fifty-three meters, in the basement of a hotel in what had once been Mayfair. The hotel had been a luxury establishment, the kind of place where the wealthy had slept in sheets of Egyptian cotton and drank champagne from crystal flutes and believed that the world would always be exactly as it was, exactly as they had arranged it to be. The basement had been a wine cellar, and the wine cellar had been flooded for so long that the bottles had become encrusted with barnacles, their labels dissolved, their contents turned to vinegar. The recording device was wedged between two racks of Chateau Margaux 1982, a vintage that no longer existed, a taste that no one would ever taste again. Kael played the recording. The same voice, the same melody, the same frequency. The song was longer this time—six minutes and twelve seconds—and the words were clearer, and Kael understood that the song was not about a world on fire. The song was about a woman who had watched her world drown and had sung while it drowned, and the singing had been the only thing that kept her alive, and the singing had been recorded and hidden and scattered across the submerged city like seeds scattered across a field that might or might not grow.

The eighth recording. There were eleven more. Kael checked her dive clock. 04:41:07. She had time.

She found the third recording at sixty-seven meters, in the vault of a bank on what had once been Threadneedle Street. The vault had been breached decades ago—the door hung from its hinges, the interior stripped of everything valuable—but the recording device had been hidden inside the wall, behind a panel that had been painted to match the concrete. The voice sang for nine minutes and fifty-one seconds, and this time Kael understood the words, and the words were about her. Not her specifically—not Kael-7, not the scavenger with the gills and the webbing and the synthesizer and the countdown—but the person she had been, the person who had studied music before the Flood, the person who had believed that songs mattered, that frequencies mattered, that the resonance between a voice and a body was the only thing that could cross the boundary between one person and another.

She found the fourth recording at eighty-two meters, in the generator room of a power station that had supplied electricity to the eastern districts of London before the Flood shorted the grid and plunged the city into the permanent twilight of the salvage economy. The fifth was at ninety-four meters, in the crypt of a church that had been built in the seventeenth century and abandoned in the twenty-first and submerged in the twenty-second and would stand for another thousand years because stone did not care about water the way steel cared about water. The sixth was at one hundred and seven meters—she needed her thermal exchange system at full capacity now, needed her pressure-resistant ribs to hold against the weight of half an ocean, needed the nictitating membranes to protect her eyes from the absolute darkness of the deep. The seventh was at one hundred and nineteen meters, in the wreckage of a submarine that had been sent to evacuate the last survivors of the Flood and had been crushed by a collapsing building before it could surface. The voice was still singing—always the same voice, always the same melody, always the same frequency—and Kael followed it deeper, and deeper, and deeper, and every recording cost her something that she had not known she still had.

She had started the dive with two hours of oxygen. She had one hour and fourteen minutes left. She had followed the signal through seven recordings, and there were five more to find, and the next coordinate was at one hundred and thirty-four meters, which was the limit of her modification tolerances, the depth at which the pressure would begin to crack her reinforced ribs and the cold would begin to freeze her modified blood and the darkness would become so absolute that her retinal overlay could not compensate for it. She checked her dive clock. She checked her oxygen. She checked the count on her forearm—seven mutations, seven recordings, seven steps toward something she could not name and could not stop approaching.

She dove.

The eighth recording was at one hundred and thirty-four meters, in the belly of a corporate transport that had sunk during the evacuation, its cargo hold full of the personal effects of executives who had been too slow to reach the arcologies. The ninth was at one hundred and forty-seven meters, in a maintenance tunnel beneath the Thames, a tunnel that had been sealed after the Flood and never reopened because there was nothing in it worth salvaging. The tenth was at one hundred and fifty-eight meters, attached to the anchor chain of a cargo ship that had been sunk deliberately to form a barrier against the rising water, a barrier that had failed, as all barriers had failed, as all barriers would always fail. The eleventh was at one hundred and sixty-nine meters, and by this point Kael could feel her ribs beginning to compress, could feel the thermal exchange system struggling to keep her blood from freezing, could feel the synthesizer in her throat producing sounds that were not words and not music but something in between, something that had never been heard before, something that was being broadcast to the corporate servers and would be analyzed and catalogued and sold.

There was one recording left. The coordinates pointed to the deepest point in the submerged city—a geological depression where the riverbed had cracked during the Flood, creating a trench that dropped to one hundred and eighty-one meters. Kael checked her oxygen. Seventeen minutes. She checked her dive clock. She checked her mutations. Seven. Seven adaptations, seven recordings, seven steps toward the person she had been before the Flood, and now one more step to complete the count and reach zero.

The final recording was at one hundred and eighty-one meters. The water at this depth was not water anymore—it was a medium, a substance, a state of matter that existed between liquid and pressure and the particular silence that comes when you are deeper than sound, deeper than light, deeper than anything that the human body was designed to experience. The recording device was sitting on the riverbed, half-buried in the silt, and when Kael picked it up and activated the playback, the voice was not a recording anymore.

The voice was hers.

Not the synthesized voice of the corporate implant, not the data stream that the servers received and catalogued and sold. Her voice—the voice of the music student who had believed that songs mattered, who had believed that frequencies could cross the boundary between one person and another, who had recorded twelve messages before the Flood and scattered them across the city for someone to find, someone who would follow the signal to its end and become the person who could hear it. The final recording was not a song. It was a question. And the question was the same question that Kael-7 had been asking since the first modification, the first mutation, the first step away from the person she had been: What are you willing to lose to find what you have lost?

Kael checked her oxygen. Eight minutes. She checked her mutations. Seven. She checked the recording—the eighth recording, the one she had found in the concert hall, the one that had started everything—and she saw the timestamp on the data transmission, and the timestamp was not from forty-seven years ago. The timestamp was from fourteen hours ago. The recording had been made that morning, by someone who was still alive, someone who was still singing, someone who was waiting at the coordinates embedded in the final transmission.

Kael-7 surfaced at the coordinates. The place was an arcology—not a corporate tower, not a salvage platform, but something older, something that had been built before the Flood by people who had believed that the water would rise and had prepared for it. The arcology was a dome of reinforced glass and carbon fiber, anchored to the seabed at one hundred and eighty-one meters, and inside the dome was a garden, and in the garden was a woman, and the woman was singing.

She was older than Kael—in her sixties, perhaps, or her seventies, the kind of age that belonged to a world that still had time, still had years, still had the option of growing old. Her hair was white and her hands were thin and her voice was the voice from the recordings, the voice at 432 hertz, the voice that resonated with the natural vibration of the human body. She stopped singing when Kael entered the dome. She looked at Kael's gills and her webbing and her retinal overlay and her synthesizer and her countdown, and she said, in a voice that was neither surprised nor frightened nor sad, "You found them."

Kael tried to speak. The synthesizer processed her attempt and produced a stream of data that meant nothing, that would never mean anything, that was only the sound of a person trying to cross a boundary that had been dissolved decades ago. The woman nodded, the way you nod when you have been waiting for someone for a long time and they have finally arrived, and she began to sing again, and the song was the same song, and the frequency was the same frequency, and Kael-7, who had followed twelve recordings through one hundred and eighty-one meters of water across forty-seven years of silence, closed her eyes and listened.

The countdown on her forearm had stopped. It did not say zero. It did not say anything at all. The synthesizer in her throat was silent—truly silent, for the first time since the seventh modification—and in the silence, beneath the woman's voice, beneath the melody, beneath the water and the pressure and the weight of everything she had lost, Kael-7 heard another voice. It was her own voice. It had been there all along.