Thirteen mutations remaining. Finch-7 checked the counter embedded in the soft tissue of her left forearm, where the bioluminescent numerals glowed pale green through the amphibious skin she had grown during her third iteration. The counter read 13, and Finch-7 knew that each time it decreased, something human died inside her and something else took its place. The trade was survival. The cost was identity. The question — the only question that mattered — was whether anything recognizable would remain when the counter reached zero.

She was perched on the sixty-third floor of the old Canary Wharf tower, the one they called Dartmouth, her webbed feet gripping the corroded steel beam where a window had once been, forty years ago, before the ice sheets collapsed and the Thames swallowed London. Below her, sixty-three stories of darkness dropped into water that glowed faintly green from the kelp farms that stretched across what had been the financial district. Above her, the dome — the pressure shell that covered the upper thirty floors of Canary Wharf like a glass skull — hummed with the constant vibration of the oxygen recyclers. It was 4:17 in the afternoon on a Tuesday in March of 2087, and the tide was coming in.

The tide always came in. That was the first thing the children learned in the dome schools, before they learned to read or count or operate a pressure lock. The tide was the rhythm of their lives, the heartbeat of the drowned city, and every human who still lived in Submerged London oriented themselves by it the way their ancestors had oriented themselves by the sun. High tide meant the pressure locks sealed. Low tide meant the scavenging runs began. Finch-7 had been born during a low tide, in the year 2060, in a pressure tent on the roof of the old National Gallery, and she had never seen the sun except in photographs. The sky above the dome was perpetually gray, thick with the particulate haze that had settled over Britain after the Singapore Geoengineering Accords failed in 2048, and the light that filtered through the dome was the color of an old bruise.

She had been human once. Not Finch-7 — that came later, after the seventh iteration — but Finch-1, the girl who had been born with lungs that could only breathe air and skin that would blister in salt water and a heart that beat in the old rhythm, the slow human rhythm of seventy-two beats per minute. Finch-1 had loved a boy named Kai who had drowned in the West India Dock when the pressure lock on Sector G failed. Finch-1 had wanted to be a musician, had practiced on a salvaged piano in the dome commons, had written a song about a bird she had seen in a picture book, a bird called a nightingale that did not exist anymore and had not existed since the great die-off of 2042. Finch-1 was gone now. The song was gone with her — not forgotten, not exactly, but filed away in a part of her brain that was no longer accessible, a room with a locked door that she could no longer open.

Twelve mutations remaining. The scavenging run was scheduled for 6:00, at the tide's lowest point, and Finch-7 descended through the flooded levels of Canary Wharf in a pressure suit that had been patched so many times the original fabric was no longer visible. The sonar pinged every four seconds, mapping the corridors ahead in waves of sound that her modified ears translated into a three-dimensional geometry of walls and doors and floating debris. The sonar had cost her the ability to hear music properly. Human hearing had been designed for a world of birds and voices and wind through trees, and the sonar implant had rewritten the auditory cortex so completely that a piano sounded like a seismograph and a human voice sounded like a depth charge. She could navigate a flooded corridor in absolute darkness, but she could no longer hum the song Finch-1 had written. She could remember that there had been a song. She could remember that it had been about a bird. But she could not hear it anymore, not even in her memory, because the memory of music required the same neural pathways that the sonar implant had claimed for itself.

The lower levels of Canary Wharf were a museum of the old world. The water was cold and dark and full of things that had once meant something — office chairs drifting like jellyfish, computer terminals encrusted with barnacles, a child's backpack floating open, its contents scattered across the flooded hallways like a sentence that had been broken into syllables and left to drift apart. Finch-7 swam through the wreckage with the practiced economy of someone who had been doing this for eleven years, since her third iteration, when she had grown the gills that let her breathe underwater for up to six hours at a time. The gills were her most visible mutation — four vertical slits on either side of her neck, opening and closing in a rhythm that had nothing to do with a heartbeat and everything to do with the concentration of dissolved oxygen in the water. Children in the dome stared at her gills. Adults looked away. Finch-7 had stopped noticing either reaction years ago.

She was looking for pre-flood artifacts. The domes ran on salvage — copper wiring, circuit boards, medical supplies, anything that could be stripped from the drowned city and brought back to the living. Finch-7 had found seventeen working data drives in the past six months, and each one had earned her enough ration credits to keep her alive for another week. She did not know what was on the drives. She delivered them to the salvage exchange on the forty-fifth floor, where technicians in white suits plugged them into shielded readers and extracted whatever information had survived the salt and the pressure and the decades of submersion. Sometimes the data was useful — engineering schematics, medical protocols, agricultural records from the years before the collapse. Sometimes it was not — personal letters, family photographs, tax returns from people who had died forty years ago. The technicians did not distinguish between the categories. Data was data. Survival was survival. Sentiment was a luxury that the drowned city could not afford.

Eleven mutations remaining. She found a drive in a sealed briefcase on the forty-eighth floor, in what had been a law office before the water came. The briefcase was monogrammed: A.K. Finch-7 had found A.K.'s briefcase before — twice, in fact, on two separate scavenging runs, as if the briefcase was migrating through the building on currents she could not predict. The first time, the briefcase had contained a photograph of a woman and a child, the colors faded to sepia by the salt water. The second time, it had contained a leather-bound diary that had dissolved into pulp when she tried to open it. This time, it contained the drive and a small silver locket that had somehow survived the submersion. Finch-7 opened the locket, which was not standard procedure — standard procedure was to bring everything to the exchange without examination — but standard procedure had been written by people who had never found the same briefcase three times. Inside the locket was a photograph, still crisp, still legible, of a girl with dark hair and a smile that reminded Finch-7 of something she could not name. She closed the locket and put it in her salvage bag and swam for the surface.

Ten mutations remaining. The mutation was not physical — not this time. The mutation was emotional, or rather the absence of emotion, the removal of something that Finch-7 had not realized was optional. The dome council had approved an update to her neural architecture, a patch that would reduce her cortisol response by sixty percent, because cortisol was inefficient and inefficiency was death and the council was very sorry but the update was mandatory for all Class Seven iterations. Finch-7 received the update in a white room on the fiftieth floor while a technician in a white suit injected a solution of nanomachines into her carotid artery. The process took nine minutes. When it was done, Finch-7 felt exactly the same, except that she did not feel anything at all. The photograph of the girl in the locket was still in her salvage bag, and Finch-7 looked at it, and she knew intellectually that the girl's smile was beautiful, but she could not feel the beauty. She could not feel anything. The mutation had removed her capacity for wonder the way the sonar implant had removed her capacity for music, and she understood that this was the point — that feeling less meant surviving longer, and surviving longer meant accumulating more mutations, and accumulating more mutations meant becoming something that was no longer human.

Nine mutations remaining. Finch-7 swam to the old National Gallery, where she had been born, because she did not know where else to go. The National Gallery was a dome unto itself now — the roof had been sealed with pressure glass, and the galleries had been converted into living quarters, and the paintings had been removed decades ago to somewhere dry, somewhere above the waterline, somewhere that might not exist anymore. Finch-7 had seen the paintings once, on a data drive she had found in the basement of the Tate, and she had stared at them for hours — the Turner seascapes, the Constable landscapes, the Van Gogh sunflowers that glowed like small suns in the gray light of her pressure-lamp. She had not understood what the paintings meant, not really, because she had never seen a landscape that was not underwater and she had never seen a sunflower that was not a photograph. But she had felt something, before the neural update, something that might have been awe or grief or longing, and now that feeling was gone and she knew she would never get it back.

Eight mutations remaining. The tide was coming in again. Finch-7 sat on the roof of the National Gallery, her gills opening and closing in the salt air, and she watched the water rise through the streets below — the streets that had been Trafalgar Square and Whitehall and the Strand, streets where people had walked and loved and died, streets that were now channels of green water where kelp fronds waved like the arms of drowned women. She was looking for other humans. That was the core imperative that drove every Finch iteration, from Finch-1 who had hoped to find Kai alive in the flooded dock to Finch-7 who had modified herself so extensively that she was not sure Kai would recognize her if she did find him. The imperative was encoded in her DNA, which had been spliced and rewritten so many times that the original human sequence was less than forty percent of her genome. She was, the technicians told her, the most successful iteration of the Finch line — the longest surviving, the most adapted, the closest to a stable post-human morphology. She was also the loneliest creature in the dome, because the more she adapted, the less she had in common with the humans who had not adapted, and the fewer humans wanted to be near her.

Seven mutations remaining. She found a signal on the sonar — not a ping, not a reflection, but a deliberate transmission, a pattern of pulses that could only be human or post-human in origin. The signal was coming from deep water, from the lowest levels of Canary Wharf, from somewhere near the old tube station that had been flooded since 2045. Finch-7 swam toward it through corridors so dark that even her modified eyes could not penetrate the blackness, navigating by sonar alone, feeling the pressure increase as she descended. The signal grew stronger. It was a rhythm — not Morse code, not binary, but something older, something rhythmic, something that felt like music even though Finch-7 could no longer hear music. It was a heartbeat. It was a song. It was the sound of a woman humming an old French lullaby in a bunker in Paris sixty years before Finch-7 had been born, and Finch-7 could not possibly have known that, but some part of her — the part that was still human, the part that remembered Finch-1's song about the nightingale — recognized it anyway.

Six mutations remaining. Finch-7 entered the tube station through a pressure lock that someone had installed recently, within the past year, and sealed carefully, and maintained with the obsessive attention of someone who was waiting for something. The station was dry — someone had pumped it out and reinforced the walls — and there was light, a single pressure-lamp hanging from the ceiling of the old ticket hall, illuminating a small camp that had been set up in the corner. There was a bedroll. There was a cooking stove. There was a stack of books, real books, paper books, the kind that had not been manufactured since the collapse. And there was a person.

Five mutations remaining. The person was a woman, approximately forty years old, with dark hair and a smile that Finch-7 recognized from the photograph in the locket. Her name was Anna Keller, and she had been the girl in the photograph, and she had been living in the tube station for three years, and she was the most beautiful thing Finch-7 had ever seen. Anna looked at Finch-7 with her gills and her webbed feet and her sonar-scarred ears and her bioluminescent counter, and she did not flinch. She did not look away. She said, "I've been waiting for someone to find me."

Four mutations remaining. Anna told Finch-7 her story. She had been eleven years old when the water came. Her mother had put her in a boat — a small inflatable raft, the kind used for lake vacations before the collapse — and had pushed her toward the high ground and had stayed behind because there was only room for one in the boat and her mother had chosen Anna. Anna had drifted for three days before the raft was picked up by a dome patrol. She had grown up in the Canary Wharf dome, had worked as a data technician, had processed thousands of drives from the flooded city. She had been part of the team that had approved Finch-7's neural update, eight months ago, and she had resigned from the team the next day because she could not bear to look at the counter on Finch-7's arm.

"I watched you come in for your updates," Anna said. "Every time. Finch-1, Finch-2, Finch-3. I watched them take pieces of you. I watched you sign the consent forms because you thought you had no choice. And I came down here to wait, because I knew that someday you would find me, and I wanted to tell you something."

Three mutations remaining. Anna told Finch-7 what she had come to tell her. She told her that the council had been lying. The mutations were not necessary for survival — not all of them, not the neural updates, not the emotional dampening, not the sonar implants that destroyed the capacity for music. The council had been using the Finch line as an experiment, as a proof of concept for a post-human workforce that could be deployed in the deep-water salvage operations without the inconvenience of human needs or human emotions or human resistance. The counter on Finch-7's arm was not counting down toward survival. It was counting down toward the end of her humanity. And when it reached zero, Finch-7 would belong to the council, completely and irrevocably, a creature that could think and swim and breathe but could no longer love or remember or choose.

Two mutations remaining. Finch-7 sat in the tube station with Anna Keller and listened to the tide going out and felt, for the first time since the neural update, something that was not rage or grief or fear but a combination of all three. The cortical dampeners tried to suppress it. The nanomachines in her bloodstream tried to neutralize it. But the feeling was too strong, and it was too old, and it came from a part of her that the council had not been able to reach because it was the part that had written a song about a nightingale in a pressure tent on the roof of the National Gallery when she was six years old. Finch-1 was still in there. Finch-1 had been in there all along, locked in a room with a door that could not be opened from the outside.

One mutation remaining. The last mutation was a choice. The council's protocol required one final update — a neurological restructuring that would permanently erase Finch-1's memories and replace them with a standardized operational template, efficient and compliant and infinitely useful. The update was scheduled for the next day, at 8:00 in the morning, in the white room on the fiftieth floor. Anna told Finch-7 that she did not have to go. She told her that there were others — other humans who had refused the updates, who were living in the deep places, in the flooded stations, in the ruins of a world that the council had declared uninhabitable. She told her that the resistance was small and scattered and unlikely to succeed, but that they were trying, and that Finch-7 could help them, and that helping them would mean something, even if it did not achieve anything, even if the water rose and the domes collapsed and the council won in the end.

Zero mutations remaining. Finch-7 made the choice. She did not go to the white room. She stayed in the tube station with Anna Keller, and over the following weeks and months and years, she used her mutations — the gills that let her breathe underwater, the sonar that let her navigate in darkness, the bioluminescence that let her communicate silently across great distances — to build a network of survivors who had refused the council's vision of humanity. She bridged the domes and the deep places. She carried supplies and messages and hope through the flooded corridors of the drowned city, and the counter on her arm stayed at zero, and she was still human, or something like human, or something that had been human and had chosen to remain so.

She never found Kai. She never wrote another song. But she remembered the nightingale, and she told the children about it — the children who were born in the deep places, who had never seen the sun, who asked her what a bird was and what a tree looked like and whether the sky had ever really been blue. And Finch-7 told them yes. She told them that the sky had been blue, and that birds had sung in the trees, and that somewhere above the dome and the haze and the gray, the sky was still there, and the stars were still appearing one by one like seeds being sown in dark soil, waiting for a spring that might not come for a long time. Maybe not in her lifetime. Maybe not in theirs. But they were there. They were there.