The water rose in 2063 and did not leave. It took the streets of London and did not come back, and the buildings that had been buildings became islands, and the islands became ships, and the ships became home to the people who had learned to live in the water. Mara was one of those people. She was twenty-six years old, or what remained of someone who had been twenty-six years old, because the water had not been kind to the body and the body had not been kind to the mind.

Mara survived. This was her primary activity. She survived by making choices. Each choice was a mutation. Each mutation was a survival mechanism that had worked once and then became part of the organism and then became part of the strategy and then became part of the person. Mara had no single person. She had a collection of survival strategies that had accumulated over eight years of living in a city that was water.

She lived on a ship that had been a ferry. The ferry had been renamed The Resilient by its last owner, and Mara had kept the name because the water liked names and it punished names that were not strong. The Resilient was a good name and the water respected it. Mara respected it too, though she would not have admitted it aloud. The water was listening and it understood admissions.

The mutation count was a number that Mara carried in her head. It was the number of times she had changed. It was not a precise number because the water of her life was not precise and the mutations were not discrete events but continuous processes, and counting them was like counting the ripples in the Thames when the Thames was a lake. But she carried the number and she updated it when she could.

The first mutation was the decision to stay when the water rose. Everyone else had gone to higher ground, which was a concept that existed in 2063 but was increasingly theoretical. The higher ground was in Cambridge and Oxford and the Chiltern Hills, and the higher ground had capacity for everyone from London. It did not. The higher ground said this. Mara heard this. The decision to stay was the first mutation. It changed something in her that was not physical. It changed something in her strategy. It changed her from a person who lived in a city to a person who lived in the water.

The second mutation was the decision to take the knife. She found the knife in a house that was half underwater, on a street that had once been called Soho and was now called Under and the water had a way of renaming things. The knife was in a kitchen. The knife was a small knife, a kitchen knife, and it was dull and it was rusted and it was the most useful thing Mara had found in three days. She took it and she carried it and she learned to use it and the knife became part of her and the knife became part of her strategy and the knife became a mutation.

The third mutation was the decision to trust no one for a week. A man named Tom had found her on the roof of a building in what used to be Westminster and had offered her food. The food was bread and it was stale and it was the best bread Mara had eaten in two years. Tom was kind. Tom had a small boat. Tom said he had a room in a building on the South Bank and the water did not reach the second floor and Mara could stay there. Mara took the bread and she did not take Tom's offer. She sat on the roof and she ate the bread and she watched Tom sail away and she felt the mutation happen in her chest, like something changing shape, like a cell dividing. The mutation was: trust no one for a week. The week became two weeks. The two weeks became three months. The three months became a year. The mutation became part of her strategy.

The mutation count rose. Mara did not count every mutation. She counted the major ones, the ones that changed something fundamental, the ones that turned her into a different organism, the ones that made her more adapted to the water and less adapted to the person she had been.

She had been a librarian in the before. This was not useful in the after. The mutation that erased the librarian was the decision to burn books for warmth. She burned them on a Tuesday. She burned a shelf of them. They were fiction, and fiction was not useful in the water, and the warmth they provided was real, and Mara sat in the warmth and she read the ashes and she learned that ash has a texture and ash is a form of writing and ash is what remains when you burn a story, and the story is gone but the ash is there, and the ash is a record, and Mara kept ash in a jar and the ash became part of her strategy.

She had been a librarian. The mutation count said: the librarian is gone. The librarian has been replaced by a water person. The water person has mutations. The water person has mutations because the water demands mutations. The water does not care about the librarian. The water does not care about anything. The water is the environment and the environment selects.

The mutations continued. She learned to swim with weights tied to her ankles. She learned to fish with her hands. She learned to read the water for signs of what was alive and what was dead. She learned to recognize other water people from the way they moved, the way their bodies had been shaped by eight years of living in water. The water people had different bodies than the land people had. They were leaner. They moved differently. They had mutations. So did Mara.

The countdown of humanity was a number she did not speak aloud. It was the number of human things she had lost. The first thing she had lost was her name. Not the name Mara, which was the name the water had given her, but the name she had been given by her mother. Her mother was dead. Her mother had been a land person and the water had taken her. Mara had not gone to the body. This was a mutation. The mutation was: leave the body in the water. The body decomposes. The body becomes part of the ecosystem. This is what the water people say. The water people are right. The mutation is part of her. She has lost her name and she has lost her mother and she has lost the land and she has lost the concept of a shelf and a book and a quiet room with no water on the floor, and the countdown is rising and the countdown is the mutation count.

She encountered land people. They came on ships that were not ferries, ships that had engines and sails and people who had stayed on the high ground and had rebuilt, and they came to the water city looking for things. They were looking for old things, for artifacts, for books, for anything that was from the before and that they could take to the high ground and display and remember.

Mara watched them from the deck of The Resilient. She watched them walk through the streets of the water city, which was the streets of London under three meters of water, and she watched them pick up objects and put them in bags and carry them to their ships, and she understood that they were collecting the remains of a civilization that they had abandoned, and they were collecting the remains to display on the high ground and to say: this is what we lost. And they were right. This is what they had lost. But the people who had stayed, the water people, the mutations, they did not have objects to take. They had mutations. They had the number in their heads that was rising every day.

One of the land people, a woman with a clipboard and a clean jacket, came to The Resilient and asked Mara if she had any books. Mara said she had ash. The woman looked at the jar and she took it and she examined it and she said, This is remarkable, and Mara understood that the woman thought the ash was remarkable because it was evidence, because it was an artifact, because it was something from the before that proved the before existed. Mara said, This is what the books became. The woman nodded and put the jar in her bag. Mara watched her walk away and she felt a mutation in her chest, the kind that changes the organism. The mutation was: I am not an artifact. I am not evidence. I am a living thing and I have mutations and the mutations are mine and they are not for display on the high ground.

The countdown of humanity continued to rise. She had lost something else, she realized, when she saw the woman with the clipboard. She had lost the ability to feel grief. Grief is a land thing. Grief is the feeling you have when you lose something on the land, when something is gone and you can go to the place where it used to be and stand there and feel the absence. Grief requires a place where things used to be. The water does not have places where things used to be. The water has only places where things are, and they are underwater, and the things are part of the ecosystem, and the ecosystem does not grieve, and Mara was part of the ecosystem.

The mutation count was high. She was mostly water now. She was mostly mutation. She was mostly the things that had been selected by the environment, and the environment was water, and water was patient, and water had selected her over eight years, and the selection had changed her in ways she could not reverse, in ways she did not want to reverse.

She stood on the deck of The Resilient and she looked at the water and she thought about the countdown and she thought about the mutations and she thought about the land people and their ships and their artifacts and their ash, and she understood that she was a different species, or close to it, or close enough that the word species was no longer useful, and she carried the count in her head and she updated it daily, and she survived, and survival was the only thing that remained, and the count was rising, and the count was humanity lost, and the count was the only record she had, because she did not write, and she did not keep notebooks, and the count was her only documentation, and the count was her only proof that she had been someone before she had become this.

The water rose. The mutations accumulated. The countdown rose. And Mara survived.