The last thing Sera remembered about being human was the smell of rain. This was important because rain still fell in Submerged London — it fell harder than ever, warm sheets of it that came in off the Channel and turned the canal-streets into churning brown rivers — but Sera could not smell it anymore. She could feel it on her skin, the weight and temperature of it, but the smell was gone along with the smell of bread and the smell of cut grass and the smell of a lover's hair on the pillow beside her in the morning. She had traded the sense of smell for gills, and the trade had been necessary, and the necessity did not make the loss any smaller.

The year was 2087. London had been a water city for twenty-three years, ever since the West Antarctic ice sheet collapsed in a single catastrophic summer and the Thames swallowed everything below the fifth floor. The skyline remained — the Shard still rose above the water like a black glass accusation, the London Eye was a rusted crown half-submerged, the dome of St. Paul's broke the surface like a whale's back — but everything below was dark and silent and full of the things that people had left behind when they fled or drowned. Sera dived into those places. She was a deep-salvage operative, which meant she descended into the drowned floors of the old city and retrieved whatever the surface markets would pay for: data cores, medical equipment, art, jewelry, the occasional human bone that a family wanted for burial. She was twenty-nine years old. She had been doing this work for five years. She had been, at various points, a marine ecologist, a grad student, a daughter, a woman who laughed easily and cried at films and felt the hair on her arms rise when someone she loved walked into a room. She was none of those things now. She was Sera-Blackwood, salvage operative, generation seven, and each generation had cost her something she could not get back.

The descent began at dawn. She stood on the boarding platform at Canary Wharf, where the old financial district had been converted into a floating market, and she checked her gear with the automatic precision of someone who had done this a thousand times. Oxygen scrubbers. Pressure gauge. Thermal lining. Knife. Salvage bag. Her gill implants, the ones she had gotten at generation one, opened and closed along her ribcage like the pages of a wet book. The sensation was no longer strange to her. The strange thing was remembering that she had once breathed through her mouth and nose like any baseline human, that the air had entered her body through two small holes in her face, that the world had once smelled of jasmine and diesel and the salt of her mother's cooking. She remembered these things the way you remember a photograph of a photograph: the information is there but the texture is gone.

Generation one had come when she was nineteen. The floods were still rising then, the evacuation still underway, and the augmentation clinics were offering gill implants to anyone who could pay or anyone who volunteered for the underwater construction corps. Sera had volunteered. She watched the surgeon make the incisions along her ribs, felt the cold burn of the bio-weld as the synthetic tissue fused with her own, spent three weeks in a tank of nutrient solution while her body learned to extract oxygen from water. When she emerged she could breathe in the flooded streets. She could stay underwater for hours. She could also no longer tell the difference between mint and cilantro, between gasoline and perfume, between the particular sweetness of her girlfriend Lena's skin and the generic salt of the sea. The day she realized this she stood in Lena's kitchen and held a jar of cinnamon to her nose and smelled nothing, absolutely nothing, and she understood that a door had closed behind her and would never open again. Lena held her while she wept, or tried to, but the tears mixed with the gill discharge and made a kind of brine that stung them both, and after a while they stopped trying to be sad about things that could not be changed. That was the first generation. Sera had been a marine ecologist then, and she told herself that the trade was worth it, that the gills would let her study the new ecosystem forming in the drowned city, that science required sacrifice. She believed this. She needed to believe this.

Generation two came at twenty-two. The salvage work demanded better vision in the dark water, where the only light came from bioluminescent algae that the corps had engineered to line the canal walls and where visibility dropped to three meters on good days. She went to a clinic in the floating district above what had been Camden Town and they injected viral vectors into her retinas that rewrote the genes for her rod cells, tripling their sensitivity, and at the same time rewrote the genes for her cone cells, flattening the spectrum until the world resolved into ninety-seven shades of gray. The procedure took six hours. She spent them awake, watching the colors drain from the world like water from a cracked basin. Red was the first to go. She watched the surgeon's scrubs fade from crimson to charcoal. Then orange and yellow followed, taking with them the warmth of streetlights and the glow of Lena's hair in morning sun. Green and blue held on longest, perhaps because they were the colors of the world she now inhabited, but eventually they too dissolved into the monochrome spectrum that would be her visual reality for the rest of her life. When she opened her eyes after the procedure, the world looked like an old photograph. Lena's eyes, which had been hazel — a word that now meant nothing — were the color of damp concrete. The roses that the floating market vendors sometimes sold, salvaged from rooftop greenhouses, were indistinguishable from the newsprint they were wrapped in. Sera could now see in water where she had once been blind. She could also not remember what her mother's face had looked like in color, and this loss was the one that stayed with her longest, the one that returned in the quiet moments before sleep when her mind replayed the catalog of everything she had surrendered.

Generation three at twenty-four: dermal reinforcement. The pressure at depth was causing microfractures in her capillaries, and the cold was leaching heat faster than her body could generate it. The solution was a subdermal mesh, a lattice of synthetic polymer woven into the deepest layer of her skin, that distributed pressure evenly and retained heat with ninety-two percent efficiency. The mesh also deadened the nerve endings in her skin to the point where fine touch became a memory. She could feel pressure and temperature and pain, but she could not feel texture. She could not feel the grain of wood or the weave of fabric or the softness of paper. She could not feel Lena's fingers tracing patterns on her back in the dark. She and Lena stopped sleeping in the same bed around this time, not because they stopped loving each other but because love, for Sera, had become a thing that happened entirely in her mind with no translation into the physical world. She could remember that touch had once been pleasurable. She could not remember the pleasure itself. She was like a musician who had gone deaf and could still read the notes but could not hear the music.

Generation four at twenty-six: the neural interface. The salvage work was getting more dangerous, requiring faster reactions and better coordination with the drone rigs that did the heavy lifting at depth. The clinic in the floating district above King's Cross installed a mesh of carbon-fiber filaments along her motor cortex that allowed her to control equipment with thought alone. The procedure was supposed to be routine. It was not. The filaments integrated more deeply than anticipated, spreading into the limbic system, into the amygdala and the hippocampus, and when Sera woke from the procedure she discovered that her emotions had been attenuated to a narrow band between mild interest and mild frustration. She could still think the words for joy and grief and love, but she could no longer feel the states they described. Lena left three months later. She stood in the doorway of their apartment in the floating district and said: I don't know who you are anymore. And Sera, watching her with the gray eyes of generation two and the deadened skin of generation three and the flattened affect of generation four, could not summon the tears that the situation demanded. She said: I understand. She meant it. That was the worst part. She meant it completely, and she could feel the shape of the loss in the same way she could feel pressure through her reinforced skin: as a distant abstract pressure that carried information but no pain.

Generation five came at twenty-seven, and it was the one she regretted most. Memory compression. The neural interface required processing capacity, and her organic brain could only hold so much data before the system slowed. The solution was a selective memory compression algorithm that identified low-priority memories and reduced their resolution to free up storage. She agreed to the procedure because she needed the interface to work and because the clinic assured her that only non-essential memories would be affected. The first thing she lost was the name of her third-grade teacher. Then the layout of her childhood bedroom. Then the sound of her father's laugh, which she had not realized was a memory at all until it was gone. Then whole years began to compress into summary files: age fourteen was reduced to three bullet points, age sixteen to a single sentence. Her mother's face, already colorless from generation two, began to lose detail. By the time she was twenty-eight she could not remember what her mother had looked like at all, only that she had had a mother, only that there had once been a woman who held her and fed her and taught her to read, and that this woman had existed and was now irretrievable. She could state these facts. She could not feel them. The algorithm had optimized her for survival in the present by deleting the past, and the past had been the only thing that made the present meaningful.

There were two more generations after five but Sera did not think about them as separate events anymore. Generation six had been the cold-resistance implant, a metabolic modification that let her body function at temperatures that would have killed a baseline human, and generation seven had been the oxygen efficiency upgrade, a mitochondrial tweak that let her stay at depth for twelve hours without surfacing. The procedures blurred together now, partly because the memory compression had erased the details and partly because she had stopped keeping track. She had stopped thinking of herself as a person who underwent modifications and started thinking of herself as an algorithm running iterations, each version more optimized than the last, each optimization purchased with a piece of the self that the optimization replaced. The algorithm did not care what it lost. The algorithm only cared about fitness, and Sera-Blackwood was very fit, and that was the problem.

The dive that morning took her to sixty meters, to the sunken lobby of a bank on what had once been Bishopsgate. The water was cold and black and full of suspended particles that her enhanced eyes resolved into silt and algae and the occasional floating fragment of something that might once have been a desk or a chair or a human being. She moved through the ruins with the drone rig following behind her, its lights sweeping across melted monitors and rusted filing cabinets and a waterlogged painting whose colors she could not see. She found a data core in what had been the manager's office, sealed in a vacuum case that had held for twenty-three years against the pressure and the salt and the slow erosion of everything. She extracted it and stowed it in her salvage bag and turned to ascend.

And it was there, in the darkness sixty meters below the surface of the new London, that Sera-Blackwood had what remained of a thought. The thought was this: she could not remember why she was doing this. She knew that she was a salvage operative. She knew that the surface markets paid for data cores. She knew the mechanical facts of her existence. But she could not remember the first decision, the original fork in the path, the moment when she had chosen the gills and the eyes and the skin and the interface and the compression over whatever she had been before. She knew that she had once been a marine ecologist. She knew that she had once loved a woman named Lena. She knew that she had once had parents whose faces were now abstract concepts. But the texture of these facts, the emotional weight, the sense of a continuous self moving through time — all of it was gone. What remained was a perfectly adapted organism moving through a hostile environment with maximum efficiency, making optimal survival decisions at every moment, and feeling nothing at all. She was the final iteration, generation seven, and generation seven had achieved what the algorithm had been working toward from the beginning: the complete dissolution of the person who had initiated the process. The fitness function had been maximized. The subject had been optimized into oblivion.

She surfaced. The platform at Canary Wharf was busy with the morning trade, vendors shouting prices, skiffs bumping against the docks, the smell of fried fish and kelp cakes that she could not detect. She delivered the data core and received her payment in ration credits and stood at the edge of the platform looking out at the water, at the gray sky reflected in the gray water, at the gray buildings rising from the gray sea, and she tried to remember what the world had looked like when it had colors and what touch had felt like when it had texture and what love had felt like when it had heat. She could not. The generations had stripped her of all of it, one adaptation at a time, and each adaptation had been necessary and each necessity had been a small death, and the sum of all those small deaths was the person standing on the platform now: alive, functional, perfectly adapted to the world that had replaced the world, and no longer recognizably human. She did not feel sad about this. She did not feel anything. That was the final gift of the algorithm, the ultimate optimization: the inability to grieve what you had become. A woman selling kelp cakes asked her if she was all right, and Sera looked at her with the gray eyes of generation two and said yes, she was fine, everything was fine, and she believed it because belief was all that remained when the capacity for doubt had been compressed and deleted and overwritten, and she walked home through the floating market in the gray rain that she could feel but could not smell, generation seven, the last iteration, perfectly alone.