1925. Florence.

The sound began in March. A low, persistent hum that seemed to come from somewhere beneath the floorboards of the apartment at 14 Myddelton Passage. Florence first noticed it on a Tuesday evening, when she was tuning the piano in Mrs. Ashworth's flat on the third floor. The piano was a Broadwood upright, 1897, with a warped soundboard and three dead notes in the upper register. Florence had been tuning pianos for seven years, since her father's hands gave out and she took over his route. She knew the sound of a piano in distress. This was not a piano. This was something else.

She stopped. She pressed her ear to the wall. The sound was there, and then it was not. Like a breath held and released. She dismissed it as the building settling. Old buildings in Islington settled constantly, groaning and creaking like elderly animals readjusting their bones.

But the sound returned the next night. And the next. It was not loud, exactly, but insistent. A frequency so low that Florence felt it rather than heard it, in the soles of her feet when she stood in the kitchen, in her jaw when she lay in bed, in the fillings of her teeth when she sat at the window reading. It was, she decided, like the sound of a cello string being tuned to a pitch that did not exist on any musical scale. A note that wanted to be played but could not find its place in the order of things.

She asked her neighbors. Mrs. Ashworth heard nothing. Mr. Pendelton on the ground floor said she was imagining things. The young couple in the basement flat, newlyweds named Mr. and Mrs. Cole, said they had not noticed anything unusual. Florence did not press the matter. She was twenty-five years old, unmarried, living alone in the apartment her mother had left her, and she knew that a young woman who complained about sounds that no one else could hear was a young woman asking to be pitied or pathologized. She said no more about it.

But she listened.

1975. Clara.

The sound began in January. A low, regular thrumming that leaked through the ceiling of the apartment at 14 Myddelton Passage. Clara first noticed it while she was making tea. The kettle was whistling, and beneath the whistle, beneath the hiss of the gas flame, there was something else. A pulse. A heartbeat. As though the building itself had learned to breathe.

She turned off the kettle. The sound continued. She pressed her ear to the ceiling. It seemed to come from the flat above, where an elderly man named Mr. Kostas lived. Mr. Kostas was Greek, eighty-three years old, and rarely left his flat. Clara had seen him twice: once in the stairwell, shuffling to the post box, and once at his window, staring at the street with an expression that suggested he was looking at something far beyond the brick walls of Islington.

Clara was twenty-five years old. She worked at a community center in Finsbury, running programs for kids who had been excluded from school. Teenagers with records, kids who smoked too much dope and stole car radios and had the faces of people who had given up on the idea that adults might help them. She was good at her job because she had been one of those kids. The community called her a youth worker. The police called her a nuisance. Her mother called her and asked when she was going to do something proper with her life.

She stood in her kitchen, the ceiling vibrating softly above her, and she thought: Mr. Kostas is playing his music again. She had heard it before, late at night, a low bass rumble that she assumed was Greek folk music or classical records played at a volume that seemed too loud for such a frail old man. She had not complained. The old deserved their comforts. She had simply learned to sleep with a pillow over her head.

But this was different. This was not music. It was a single sustained note, held for minutes at a time, with no melody, no variation, no rhythm. A drone. A hum. A question mark made of sound.

1925. Florence.

April. The sound grew more distinct. Florence began to track it, the way her father had taught her to track a piano's temperament: by listening for the beat frequencies between two notes that were almost but not quite in harmony. She discovered that the sound had a pattern. It was not constant. It increased in intensity around midnight and faded toward dawn. It was strongest in the corner of the bedroom that faced the street, weakest in the kitchen at the back of the flat.

She bought a tuning fork. A C-512, the standard for piano tuning. She held it to the wall and struck it and listened. The fork rang true. But beneath the ring, beneath the clean mathematical certainty of 512 vibrations per second, there was the other sound. The hum. The note that did not correspond to any pitch on the fork.

She began to keep a journal. Small brown notebook, purchased at a stationer's on Upper Street. She wrote down the dates and times when the sound was strongest. She drew diagrams of the building, marking the places where the hum was most intense. She noted the weather, the phase of the moon, the position of the tides on the Thames, though the river was a mile south and she could not see how it might matter.

On the fifteenth of April, the sound was so strong that she could not sleep. She lay in bed with her eyes open, feeling the vibration in her skull, wondering if she was going mad. She had read about women who heard things. Women whose nerves gave out. Women who were sent to rest homes in the countryside where they were given milk baths and bromide and told to think pleasant thoughts. She did not want to be one of those women.

In the morning, she went to the piano factory where she worked. She told her supervisor, Mr. Hargrove, that she was having trouble with a persistent ringing in her ears. Mr. Hargrove was a practical man who did not believe in ailments that could not be seen. He suggested she drink less tea.

That evening, Florence walked to the British Museum Reading Room. She spent two hours in the music section, reading about acoustics and resonance and the physics of sound. She learned about sympathetic vibration, the phenomenon by which one object vibrates in response to the vibration of another. She learned about standing waves. She learned that some frequencies could travel through solid objects, through brick and stone and timber, as easily as through air.

She returned to Myddelton Passage with a new theory. The sound was not in her head. It was in the building. Or beneath the building. Or traveling through the building from somewhere else. She needed to find the source.

1975. Clara.

February. The sound was driving her mad. Not because it was loud, but because it was irregular. Some nights it was there, a steady low thrum that vibrated through the floorboards and into her bones. Other nights it was gone, and the silence felt wrong, as though the building had stopped breathing.

She asked the landlord, a man named Mr. Finch who owned half the properties on the street and lived in a house in Highgate with a wife who did not work. Mr. Finch said he had not received any complaints from the other tenants. He said maybe it was the pipes. He said maybe she should open a window. He said, in the tone of a man who had said it many times before, that young people these days were too sensitive to the normal sounds of city life.

Clara did not believe it was the pipes. She had lived in London her whole life. She knew the sound of pipes. This was not pipes. This was something deliberate. Something coming from above.

She knocked on Mr. Kostas's door. She had to knock three times before she heard movement inside. The door opened a crack, held by a chain. One brown eye peered out at her from a face that looked like a map of a country that no longer existed.

Yes, he said. His voice was thin but clear.

I am sorry to bother you, Clara said. I live downstairs. I wondered if you could tell me what that sound is. The one at night.

Mr. Kostas looked at her for a long time. The chain rattled. The door closed. Clara waited. The door opened again, fully this time. Mr. Kostas stood in the doorway in a wool cardigan that had been mended in three places. He gestured for her to enter.

His flat was dark and neat. The curtains were drawn. The furniture was old but clean. On a table by the window sat a record player with a stack of vinyl beside it. Orthodox chants. Greek folk music. And something else. A single record with no label, lying on top of the stack like a secret.

Mr. Kostas walked to the record player. He lifted the needle. He placed it on the unlabeled record. A sound emerged. Low. Sustained. A single deep note that seemed to vibrate in Clara's chest.

You hear it, Mr. Kostas said. It was not a question.

Yes.

He turned off the record player. My son made that recording, he said. Before he died. In 1972. He was a sound engineer. He worked for the BBC. He said he discovered something. A frequency. A note that should not exist.

Should not exist, Clara repeated.

It is not on any scale. It is not produced by any instrument. My son found it in the basement of Broadcasting House, in the archives of old recordings. A recording of a recording. He said it had been there for fifty years and no one had noticed it.

Fifty years, Clara said. That would mean—

1925, Mr. Kostas said. The first recording was made in 1925. Somewhere in London. A woman. A piano tuner. She was trying to find the source of a sound in her building. She recorded it. Her recording ended up in the BBC archives. My son found it. He re-recorded it. He studied it. He said it was the most beautiful and terrible thing he had ever heard.

What happened to him?

Mr. Kostas looked at the record player. He died, he said. The sound took him. Not in a way I can explain. But he was never the same after he found it. He listened to it too many times. It changed him. The frequency changed his brain.

Clara stood very still. The flat was silent. The record sat on the turntable.

Why do you play it? she asked.

Because it is all I have left of him, Mr. Kostas said. And because I want to understand what he understood. I am old. I have nothing to lose.

1925. Florence.

May. The sound had a source. Florence found it after weeks of systematic investigation, using the methods her father had taught her for isolating the source of a sympathetic resonance in a piano. She moved through the building, room by room, floor by floor, pressing her ear to walls and floors and ceiling, mapping the intensity of the vibration like a cartographer mapping an unmapped country.

The source was the basement. Specifically, the southeast corner of the basement, where a iron pipe emerged from the floor and disappeared into the wall. The pipe was old, probably part of the original building's drainage system, long since disconnected from any modern use. Florence pressed her ear to the pipe and felt the vibration travel through the iron and into her skull. The sound was loudest here. This was where it entered the building.

She did not know what to do. She told no one. She could imagine how it would sound: A young woman, unmarried, claiming that an iron pipe in the basement was singing to her. She would be sent to the rest home before she could finish the sentence.

Instead, she recorded it. She borrowed a phonograph from a friend at the factory, a machine used to record the tonal quality of finished pianos. She carried it down the basement stairs, set it up beside the pipe, and let it run for three hours. The recording was imperfect, full of surface noise and the rumble of the city above, but when she played it back, the sound was there. A low, constant hum, more felt than heard, like the memory of a note that had been played in a room long after the musician had left.

She kept the cylinder. She wrapped it in cloth and stored it in a box at the back of her wardrobe. She did not know what to do with it. She did not know what it meant. She only knew that the sound was real, and that it was coming from somewhere deeper than the basement, somewhere beneath the building, beneath the street, beneath the city. A frequency that had been there all along, waiting for someone to hear it.

1975. Clara.

March. Clara went to the BBC archives. She used her community center connections, a friend of a friend who worked in the sound library. She asked about a recording from 1925, a phonograph cylinder, a woman's voice describing a sound in a building in Islington.

The archivist, a man named Derek with thick glasses and a tweed jacket that smelled of cigarette smoke, found it in three days. He called her at the community center. Miss Ayres, he said, I have something you will want to see.

Clara took the afternoon off. She went to Broadcasting House. Derek led her to a small room in the basement, lined with shelves of tape reels and cylinders and lacquer discs. He handed her a box. Inside was a phonograph cylinder in a cloth wrapper. Attached to the box was a note, yellowed and brittle, written in a precise feminine hand.

The cylinder is a recording of a persistent low-frequency sound emanating from the basement of 14 Myddelton Passage, Islington, London. Recorded May 15, 1925. The sound is audible only under specific conditions and appears to be strongest during the hours of midnight and dawn. I have attempted to identify its source but have been unsuccessful. I believe it may be of geological origin. I have named it the Myddelton Tone.

The note was signed: Florence Ayres, Piano Tuner.

Clara's hands were shaking. She read the name again. Florence Ayres. Her grandmother. The grandmother she had never known, who had died in 1927, three years before her mother was born. The grandmother she had heard stories about: a piano tuner, unmarried, independent, a woman who had lived alone in an apartment on Myddelton Passage and who had been found dead at the age of twenty-seven, cause unknown, the coroner's report citing cerebral congestion, which meant nothing and everything.

Derek looked at the cylinder. Do you want to hear it? he asked.

Yes, Clara said.

They set up the playback equipment. Derek fitted the cylinder onto the mandrel. He lowered the stylus. The machine hissed and crackled. And then the sound emerged. Low. Steady. A note that seemed to come from the center of the earth.

Clara listened. She felt the vibration in her chest, in her teeth, in the space behind her eyes. It was the same sound she had heard through her ceiling. The same sound Mr. Kostas had played on his record player. The same sound her grandmother had recorded fifty years before.

She understood now. The sound was not in the building. The building was an antenna. The sound was coming from somewhere else. Somewhere deeper. And it had been there, constant and unchanging, for at least fifty years, passing through her grandmother's basement and through the archives of the BBC and through the record player of a grieving old man and through the ceiling of her own flat, reaching across time and space and changing everyone who heard it, not because it was malevolent but because it was true. A frequency that the world had learned to ignore. A note that refused to settle into any known scale.

She asked Derek to make her a copy. He did. She took it home. She did not play it. She placed it on her shelf, beside the photograph of her mother and the key to her grandmother's apartment, which her mother had given her when she moved in. And she sat in the silence of her flat at 14 Myddelton Passage and listened to the sound that no one else could hear, the sound that had been there all along, the sound that had called to her grandmother and to Mr. Kostas's son and to her, and she understood that it was not a message. It was not a warning. It was not a song. It was simply the voice of the earth, speaking in a frequency too low for most human ears, patient and eternal and utterly indifferent to whether anyone was listening.

And she listened anyway. Because that was what you did when you heard a note that would not settle. You listened. You followed it. And if it led you to a basement, you went down. And if it led you to an archive, you searched. And if it led you back to yourself, to your grandmother, to a woman you had never met who had heard the same sound in the same building fifty years before you, you accepted the connection. You accepted that some frequencies pass through time unchanged. And you let them carry you.