Silas Dredge had been fully human once, but that was before the Thames swallowed London and before the domes went up and before the screaming started in the deep channels where the old Underground tunnels had become pressurized tombs. He remembered being human the way a man remembers a photograph of his childhood — with a kind of distant affection, unconnected to any feeling that still functioned inside the altered architecture of his chest. The photograph showed a boy standing on Westminster Bridge in the year 2061, the year the flood walls failed and the water rose thirty meters in a single week and the city that had stood for two thousand years became a submerged labyrinth of glass domes and pressure locks. The boy in the photograph had brown eyes and unmodified lungs and a face that registered emotions in the old mammalian pattern. Silas Dredge had not seen that boy in twenty-six years, and if he saw him now he would not recognize him, and this was not a metaphor. His eyes were not brown anymore. They were compound lenses, hexagonal arrays of photosensitive crystal that had been spliced into his optic nerves by a myomar surgeon in the Canary Dome when Silas was seventeen years old. The operation had cost him his natural vision and his ability to cry. He had not wept since. The ducts were still there, vestigial, but the nerves that would have triggered them had been repurposed for low-light amplification and thermal imaging and the six additional spectral bands that the compound lenses could now process. He saw more than any human had ever seen, and he felt approximately nothing about it. That was the trade. It was always the trade.

Humanity remaining: ninety-eight percent.

The percentage was a fiction, but it was a useful fiction, and Silas Dredge had been tracking it for twenty-six years. He had begun the tracking at the age of twelve, in the months after the flood, when the Westminster Dome was still under construction and the survivors were living in floating camps on the surface of the new sea. Someone had given him a waterproof notebook, a child's gift, and he had written the number 100 on the first page and underlined it twice. He did not remember why. Perhaps it had been a promise. Perhaps it had been a warning. The number had become a ritual, a kind of private ledger in which he recorded the cost of every modification, every splice, every procedure that took him further from the mammalian baseline and closer to whatever he was becoming. He knew the cost before he paid it. That was the rule. You could modify yourself into anything, into a predator or a scavenger or a piece of living architecture that breathed methane and exhaled nothing, but you had to know the cost. You had to write it down. You had to watch the number fall.

It was now the spring of 2087, which was to say it was the season when the algae blooms turned the upper waters green and the filtration systems in the public domes choked on the biomass and the air took on the faint sweet smell of organic decay that everyone pretended not to notice. Silas Dredge was thirty-eight years old and he lived in a pressurized habitation cylinder on the fifteenth level of the Greenwich Arc, which was one of the deepest residential domes in the submerged city. The cylinder was four meters long and two meters wide and it was heated by a thermal exchange coil that drew warmth from the volcanic vents on the seafloor three hundred meters below. The rent was twelve hundred hydro-credits per month, payable to the Arc Administration, and Silas paid it by working as a deep-salvage operator for a corporation called Abyssal Recovery Limited, which had its offices in a converted submarine pen near the old Docklands.

His job was to swim. Not to dive, in the old sense of the word — diving implied a return to the surface, a tank of compressed air, a time limit. Silas Dredge did not dive. He swam, continuously, for hours and sometimes days, through the flooded streets and buildings and tunnels of the submerged city, recovering salvageable technology from the ruins of the pre-flood world. He swam at depths that would have killed an unmodified human in seconds. He swam through darkness that his compound eyes read as a rich tapestry of thermal signatures and ultraviolet reflections. He swam past the drowned cathedrals and the collapsed bridges and the rusting skeletons of the double-decker buses that still lined the old thoroughfares like the vertebrae of some enormous extinct animal. And when he found something valuable — a functioning server rack, a cache of rare-earth metals, a sealed datacore from a pre-flood laboratory — he brought it back to the surface and collected his commission.

The commissions were small and the work was steady and Silas Dredge had been doing it for eleven years without complaint or ambition. He was good at it. His modifications made him good at it. And if each modification cost him another percentage point of humanity, another fraction of the boy on Westminster Bridge, the cost had always seemed reasonable measured against the alternative. The alternative was death, and Silas Dredge did not want to die. Or rather, he wanted not to want to die, which was the same thing in practice but different in principle, and the distinction had kept him moving through the dark waters long after most of the other deep-salvage operators had retired or drowned or simply disappeared into the lower domes where the unmodified went to wait for the end.

Humanity remaining: ninety-five percent.

The retinal splice had been the first modification, at seventeen, and it had cost him three percentage points. Two years later, at nineteen, he had undergone pulmonary reinforcement — the surgical implantation of synthetic alveoli that could extract oxygen from water, not efficiently enough to eliminate the need for air entirely, but enough to extend his dive time from minutes to hours. The alveoli were manufactured from a cultured collagen matrix that his body did not reject. They were pink and delicate and they looked like something that belonged inside a jellyfish, and they had cost him four percentage points.

Humanity remaining: ninety-one percent.

At twenty-three, he had commissioned the webbing. The surgery was performed in a back-alley clinic in the Southwark Dome by a former marine biologist who had lost her license for unauthorized experimentation on cephalopods. She cut the skin between his fingers and toes and grafted in sheets of synthetic membrane that could stretch and contract with the movements of his hands. The procedure was illegal and unlicensed and it hurt more than anything Silas Dredge had ever experienced, but when it was done he could move through the water at nearly twice his previous speed. The webbing cost him six percentage points.

Humanity remaining: eighty-five percent.

The gills came at twenty-seven. They were the most radical modification he had undertaken, and they nearly killed him. The surgeon in the Canary Dome — the same myomar specialist who had done his eyes — had to remove four of his ribs to make room for the branching cartilage structures that would serve as the primary oxygen exchange interface. The surgery took eleven hours, and Silas spent three weeks in recovery, and when he emerged from the medical bay his torso was permanently narrower than it had been, his silhouette subtly wrong, his breathing accompanied by a faint clicking sound that never entirely went away. But he could breathe underwater. Fully, permanently, without assistance. He could swim at any depth and for any duration. He had become something new.

Humanity remaining: seventy-eight percent.

The cost was higher than he had expected. The gill implantation had required systemic modifications to his circulatory system, his blood chemistry, his metabolic rate. The surgeon had explained this to him afterward, with the detached professional courtesy of someone who had done this procedure many times and had stopped thinking of it as remarkable. You are not human anymore, the surgeon had said. Not entirely. Your biological classification has shifted. You are now a hybrid organism, Homo aquaticus modifacta, capable of sustained submersion in saline environments at pressures up to twelve atmospheres. Your legal status has changed. There are forms you will need to file.

Silas Dredge had filed the forms. He had submitted the genetic assays and the metabolic scans and the certification of species reclassification. He had become, in the eyes of the Westminster Administration and the Canary Dome Authority and the various corporate entities that governed the submerged city, a different kind of being. He was still a citizen. He still paid taxes. He still had rights, on paper. But he was not human, not in the legal sense, and he was becoming less human every year.

He did not mind. The modifications were necessary. The deep salvage work was becoming more dangerous. The ruins were getting older, more unstable. The currents were shifting as the seafloor vents opened and closed. And there were things in the deep water now, things that had not existed before the flood — creatures that had evolved in the submerged city, hybrid organisms that fed on the refuse of the drowned civilization and had grown large and aggressive in the nutrient-rich darkness. The salvage operators called them screamers, because they emitted a high-frequency vibration that registered on sonar as a sound like a woman screaming, and they hunted in packs, and they had killed seventeen operators in the past three years. Silas Dredge had survived because he was fast and because he was careful and because he had modified himself beyond the point where screamers considered him prey. He did not smell like a human anymore. He did not register on their sensory apparatus as something edible. He was outside the food chain that had consumed seventeen of his colleagues, and that was worth any number of percentage points.

Humanity remaining: seventy percent.

The memory degradation began in his thirty-first year. It was a side effect of the metabolic acceleration required to sustain the gills and the synthetic alveoli and the modified circulatory system. His body was processing oxygen at a rate that placed enormous stress on his neural architecture. The neurons were burning too hot. The synaptic connections were degrading. The surgeon in the Canary Dome had warned him about this, in the clinical language that surgeons used when they wanted to communicate danger without creating liability. There may be cognitive effects, the surgeon had said. Short-term memory loss. Difficulty with abstract reasoning. Reduction in emotional affect. These effects are typically manageable. Most patients adapt.

Silas Dredge had adapted. He had started keeping a log — a simple text file stored on a waterproof datachip that he wore around his neck — recording the things he could no longer trust himself to remember. His mother's face. The name of his first school. The words to the song that had played on the radio during his twelfth birthday, the last birthday before the flood. The log was a kind of external memory, a prosthetic for the parts of himself that were being systematically erased. He consulted it every morning, like a religious text, and every morning he discovered something he had forgotten.

Humanity remaining: sixty-one percent.

At thirty-four, he underwent the cortical interface procedure. A mesh of synthetic neural filaments was inserted into his cerebral cortex through a series of micro-incisions in the skull. The mesh was connected to a processing unit the size of a thumbnail, implanted at the base of his brainstem. The unit gave him direct neural access to the submerged city's data network, the fragmented descendant of the old internet that still functioned in the domes and the upper water levels. He could query information directly, without speech or interface devices. He could see data as an overlay on his visual field. He could communicate with other modified humans through burst transmissions of compressed neural code. The procedure cost him nine percentage points and most of his remaining dreams. After the implant, he stopped dreaming in images. He dreamed in data streams, in hexadecimal strings, in the cold and orderly logic of machine language.

Humanity remaining: fifty-one percent.

This was the year that Gabriel Morgen's offer arrived. Gabriel Morgen was the chief executive of Abyssal Recovery Limited and he had been Silas Dredge's employer for eleven years. He was a thin man in his seventies who had been born before the flood and who had adapted to the new world by refusing to acknowledge that anything had changed. He wore suits of pre-flood cut and he spoke in the accents of a vanished London and he ran his salvage operation from an office that had been furnished to look like the study of a Victorian gentleman. He had never undergone a single modification. He was one hundred percent human, and he intended to remain so, and his refusal to adapt was either a form of courage or a form of insanity depending on who you asked.

The offer was simple. Abyssal Recovery had located an intact pre-flood research facility in the deepest section of the submerged city, a complex of pressurized laboratories that had been operated by a biotech corporation called Chimera Genomics in the years before the flood. The facility was located at a depth of two hundred and eighty meters, in a section of the old city that had never been salvaged because it was too deep and too dangerous and too close to the screamer breeding grounds. But the facility contained something valuable — a datacore with the complete research archives of Chimera Genomics, including genetic sequencing data that was worth, by Gabriel Morgen's estimate, approximately forty million hydro-credits on the open market. Silas Dredge's commission on the recovery would be two million.

He took the job. He had no reason not to. Two million hydro-credits represented more money than he had earned in his entire career. It was enough to buy a habitation cylinder in the upper domes, where the water pressure was lower and the air was cleaner and the unmodified still walked the streets in something like the old manner. It was enough to stop working, to stop modifying, to live out the remainder of his life in a state of relative peace. It was enough, Silas Dredge thought, to preserve whatever percentage of humanity he had left.

He was wrong. He was wrong about everything.

The descent took eighteen hours. Silas Dredge swam alone, following a navigation route that had been plotted by Abyssal Recovery's cartographic division. He passed through the ruins of the City of London — the flooded banking halls, the collapsed towers, the great dome of St. Paul's Cathedral that had imploded in the second year after the flood and now lay in pieces on the seafloor like the shell of a broken egg. He swam past the screamer nests, the gelatinous masses of eggs and larvae that clung to the submerged buildings, and the adult screamers tracked him with their sonar but did not attack. He did not register as prey. He was grateful for this. He was grateful for every modification that had removed him from the food chain.

At a depth of two hundred meters, the water was cold enough to challenge even his modified circulatory system. The cold seeped into his bones, into the synthetic alveoli, into the places where the neural mesh pressed against his cerebral cortex. He began to feel something that he had not felt in many years — a kind of awareness of his own body, the strange and hybrid vessel that he had built around himself. He was not human anymore. He had known this for a long time, but knowing and feeling were different things, and the cold made him feel it. He was a collection of modifications and compromises and trade-offs, a creature that had been assembled from necessity rather than design, and he was swimming into a darkness where nothing he had been would help him.

Humanity remaining: forty-two percent.

He reached the Chimera Genomics facility at midnight, which was an arbitrary concept at a depth where no light had ever penetrated, but the clock on his neural interface still tracked surface time and it registered 00:00 exactly when he found the entrance. The facility was a cluster of domes connected by pressurized tunnels, built into the foundations of a pre-flood office tower. Most of it had collapsed. One dome remained intact — the deepest one, the one that had been designed to withstand the pressures of deep-water research even before the flood. Silas Dredge found the pressure lock, cycled through it, and entered the dome.

The air inside was stale and cold and tasted of metal. The emergency power was still functioning, powered by a geothermal tap that had been designed to operate for centuries without maintenance. The lights were on. The computer systems were running. And in the center of the dome, suspended in a cylinder of preservation fluid, was a creature that Silas Dredge recognized, with a shock that cut through even the blunted architecture of his emotions, as himself.

Not literally himself. Not his body, not his face. But the creature in the cylinder was a hybrid organism, a fusion of human and aquatic DNA, and it had been designed by Chimera Genomics in the years before the flood as a prototype for a new species of deep-water salvage operator. It had gills. It had compound eyes. It had webbed fingers and toes. It had a cortical interface port at the base of its skull. It was, in every particular, a mirror of what Silas Dredge had become — except that it had been grown rather than modified, engineered from conception rather than assembled through successive surgeries, a being designed for the deep water from the first moment of its existence.

Silas Dredge stood in the cold dome and looked at the creature in the cylinder and understood, with a clarity that was indistinguishable from pain, that he was not unique. He was not even original. He was the product of a design specification that had been written by a corporation twenty-six years before he had undergone his first modification. Every procedure, every splice, every trade of humanity for survival — all of it had been predetermined. He had been walking a path that someone else had laid out, following a blueprint that someone else had drawn, becoming something that someone else had imagined. He had never made a choice. He had only followed the logic of the design. The design had been waiting for him. The design had always been waiting for him.

Humanity remaining: thirty-four percent.

He did not retrieve the datacore. He did not collect his commission. He stood in the dome and looked at the creature in the cylinder and felt something that his neural interface could not process and his modified emotional architecture could not categorize. It was grief. It was the thing that he had been trading away, percentage by percentage, for twenty-six years. It was the thing that the modification surgeons had told him was manageable, adaptable, incidental to survival. It was the thing he had written down in his log and had then forgotten, because forgetting was easier than remembering, because remembering was the one thing that made the deep water unbearable.

He swam out of the facility and into the darkness and he did not stop swimming until he reached a place where the pressure was too great for even his modified body to sustain, and then he stopped and floated in the black water and waited.

Humanity remaining: twenty-seven percent.

The screamers found him at dawn. He heard their sonar — the high-frequency vibration that sounded like a woman screaming — and he did not move. He did not fight. He did not activate any of the modifications that had protected him for so many years. He simply floated, and waited, and tracked the falling number.

Humanity remaining: twenty-one percent.

The first screamer bit into his left arm and tore away a section of the webbing between his fingers. The pain was extraordinary, a white-hot flare that registered on every nerve his modifications had not yet eliminated, and Silas Dredge felt it and did not resist.

Humanity remaining: sixteen percent.

The second screamer took his right leg below the knee. The third took a piece of his torso, the section where four of his ribs had been removed to make room for the gills. The water was full of blood now, his blood, the modified blood that could carry oxygen at pressures that would kill an unmodified human, and the screamers were feeding.

Humanity remaining: twelve percent.

Nine percent.

Six percent.

Four percent.

The neural interface was still functioning. The cortical mesh was still transmitting data. The overlay on his visual field was showing him the falling number, the last thing he would see as his compound eyes dimmed and failed.

Two percent.

Silas Dredge thought of the boy on Westminster Bridge. The boy in the photograph, the one with the brown eyes and the unmodified lungs and the face that registered emotions in the old mammalian pattern. The boy had been fully human once, and then the water had risen and the domes had gone up and the modifications had begun, and the boy had been replaced piece by piece by something that was not a boy and not a man and not a creature. The boy was a ghost now, a memory that Silas Dredge had written in his log and had then forgotten, and in the last moments before the number reached zero, Silas Dredge tried to remember his mother's face.

He could not.

The photograph was gone. The memory was gone. Everything was gone except the number, and the number was falling, and then it was not.

Humanity remaining: 0.5%.

The screamers finished their feeding and swam away into the darkness. The deep water closed over the place where Silas Dredge had been, and the cold currents carried what remained through the flooded streets and the collapsed buildings and the ancient tunnels of the drowned city, dispersing the last fragments of his body through the ecosystem that had consumed him. And somewhere in the Canary Dome, in the office of Abyssal Recovery Limited, Gabriel Morgen noted the loss of his best salvage operator and filed an insurance claim and began searching for a replacement, and the replacement would follow the same path through the same modifications toward the same ending, because the design was the design, and the design did not change, and the design had never been about survival at all.