The building of the Department of Oral Hygiene stood on a side street, where autumn leaves formed a soft carpet on the pavement and moisture rose from the asphalt like steam from a cup of mint. It was quiet here after hours. Too quiet for a place where, by day, mammals roar, turbines hum and patients laugh with paste on their lips. Marta Luczak turned the key in the laboratory door and smelled the familiar scent of eugenol, alcohol and something she had always associated with cleanliness: fresh menthol. Her finger touched the panel of the polymerisation lamp and the light illuminated her desk, confocal microscope and row of sterile instrument trays. She was in her thirties, with a comfortable apron, her hair tied up in a tight bun and a habit of order bordering on ritual. Before she touched the test tubes, she washed her hands twice: once for herself and once for the patient, the way she had once been taught by her grandmother, a doctor who brushed her teeth at the kitchen window and hummed Strauss waltzes. Either her trophies or her troubles were waiting on the countertop - it depended how you looked at it. Four slides with plaque samples, collected that afternoon: from a musician, a yoga trainer, a barista and a senior firefighter. Each was instructed: gentle sweeping motions, twice a day for two minutes at a time, fluoride toothpaste, daily cleaning of interdental spaces with floss or toothbrushes, let the tongue not be left out. Everyone nodded in agreement. And then life did its own thing. Martha sat down, put on her safety goggles and turned on the microscope. Green and blue specks appeared on the screen - bacteria in fluorescent colours, like a night city seen from above. She moved the image, zoomed in, zoomed out, a steady hand over the trackball. In the sample marked 'IWO - 21:15', something stopped her. The biofilm structure was arranged not in random streaks, but in a rhythm. There were more specks where there should have been a pause, and fewer in places that looked like notes. An illusion? She wiped the screen. A phone played on the undercurrent, a protracted notification ringing. Iwo Grot - the guitarist from the neighbouring club - had sent a voice message: he said he had been feeling a strange metallic aftertaste for the past two days, that he had gone back to the rinse liquid for a quick one because he had played a long time and didn't have the strength to floss, and that he would drop by tomorrow if he could find a moment before rehearsal. His voice was muffled, as if he were sitting in an empty room full of instruments that were silent. - A metallic aftertaste... - Martha muttered and wrote down a few sentences on the form. She looked at the screen again. The rhythm was not disappearing. The biofilm was responding gently to the light of the microscope like skin to sunlight. She adjusted the filter. The droplets seemed to tremble. Out of habit, she quietly switched on the metronome on her phone, 60 beats per minute, then 80, 120. When she switched to 440 Hz the rehearsal sound of the reed she had in her app since playing the viola, the diaphragm of the ultrasonic scaler on the adjacent table vibrated slightly. At the same time, the image on the monitor moved, as if something on the slide had responded: a tiny, almost invisible displacement, a thickening of dots at equal intervals. In the corridor, an old clock chimed twenty-two. Oskar, the dental technician, slipped his head through the door without knocking. - Are you working nights again? - He asked, holding up a mug of ginger tea. - 'Marian says the lock after eleven is blocking the lift, so if you're going down to the basement, it's now or tomorrow morning. - I'm not going down. 'Not yet,' replied Marta, staring at the screen. - Look. See? It doesn't look random. Oskar put down his mug and stepped closer, sliding his mask down over his chin. He shook his head and then laughed briefly, without conviction. - Maybe you need sleep and not a confocal? A spot is a spot. You know, bacteria like to dance in clusters. - Not in a score like that,' she whispered. - Iwo talks about a metallic aftertaste. And here I see the answer to the 'A' sound. - 'Do a check,' threw Oscar the most practical advice he could afford at this hour. - Different reed, different tone, different sample. Martha reached for the kit: a tuning fork with 523 Hz, a plastic thread box made into a trinket container, a size 0.6 mm interdental brush. She set the microscope to the barista sample. Nothing. For the trainer's sample. A slight tremor, but chaotic. On the fireman. Silence, like a forest clearing after rain. She returned to "IWO - 21:15", touched the reed to the tabletop, a clean wave went over the metal, and then the mammal in the corner of the office gave a low murmur, as if someone had set it to the "deep ocean" programme. - 'This is not normal,' said Oskar matter-of-factly, as he guarded normality like a catalogue of crowns. - A mammal does not react to sounds. Unless... - Unless the hose is clutching - Marta finished and stood up. She put her hand to the suction hose. The vibration was almost imperceptible, but goosebumps went from her fingers to her shoulder blades. - It's coming from below. From the amalgam separator. Marian mentioned that it was leaking again. The lab lights dimmed for a second, the diodes of the polymerisation lamp blinking in a sequence that resembled the light signal from a torch. Marta and Oskar looked at each other. - 'If it's a short circuit, then you need to cut the power,' said Oskar, already clutching his sweatshirt. - 'And if it's... something else, I don't want to be left alone with this. Martha tucked the slide with Iwo's sample into the portable incubator, secured it with tape and slipped it into her apron pocket, still writing in her notebook: "Biofilm rhythm - response to A4 = 440 Hz. Suction machine + separator?" Then she rapped a headlamp, nitrile and a small spatula, as if there was not electricity but a patient waiting below. The corridor was long and cool, the old terracotta floor reflecting the light of the motion sensors. A prevention poster hung from the ceiling: smiling faces, a slogan about the power of an earned habit. Martha smiled half-heartedly. Habits kept the day in check. The night was claiming its own. The basement smelled of wet plaster, dust from models and something metallic associated with a spoon scalded in boiling water. Marian, the security guard, sat in his cloister with his radio and crossword puzzle, nodding to them from afar. - 'It's bubbling down there again,' he muttered. - As if someone was blowing through a straw. And you know I don't like any ghosts. - It's not ghosts, it's the plumber - Marta reassured him, although she herself felt the words clatter like porcelain against the tabletop. - We'll check right away. The door to the separator room was old, remembering the days of amalgams mixed on stones and tissue paper. She touched the handle: cool, slightly damp. From behind the door came a low, steady sound, like the modulated purr of an icy fan. Her chest trembled in its tact, as if the sound held her by the bones. - Do you hear? - she asked, and answered herself, as Oskar was already slipping his phone out of his pocket, apparently calculating how much a new separator would cost. The headlamp cut the gloom as she swung the door open. Inside it was flickering. Someone had hung air bells on the drunken table - but these were no ordinary metal tubes. Thousands of used interdental brushes, miniaturised into one eye and a wire, were linked in rows like strings of hake. They trembled gently, bumping against each other, producing tones that were surprisingly pure. There was no draught to be seen anywhere. - 'Who did this...' whispered Oskar, and the words were left halfway. Martha took a step further in. Separators, tanks, old hoses - everything looked ordinary until she realised that the sound had a rhythm identical to the one she saw on the microscope screen. She lifted her hand and stuck the sample incubator to the metal housing of the separator. Almost imperceptibly, but clearly to her fingers, the plastic wall of the incubator responded with a tapping sound to each beat of the wave. The glaze on the picture in her head - white, shiny like a cabochon - sparkled with the thought that sound doesn't just carry through the air, but picks its way through whatever is harder. On the shelf beside her lay an old roll of dental floss, long since untangled by a student who hated it when something was 'too even'. Now the floss rose slightly, as if someone was air writing in slow motion. The string trembled and formed into a single word: LISTEN. Martha bit her lower lip until she felt a dry reminder: don't do that, teeth don't like it. She took a breath. She brought her fingers closer to the string of toothbrushes, like an instrument she was just learning. At the same moment, the lights on the ceiling flashed once more and the ventilation system sucked in air so violently that she felt briefly dizzy. - 'If this is a joke, then someone has great taste in music,' Oskar mumbled, but he stood still, as if afraid that one careless move and something would break. - Do you have it recorded? Marta reached into her pocket for her phone. Before she could unlock the screen, the sound picked up for a second, jumped to a higher pitch, and then a darker wedge broke away from the corner where the shadow of a large wardrobe of forms lurked. The shape was sharply defined, as if someone had cut it out of the night: the profile of a perfect incisor, magnified and still for a fraction of a second before....