A mist hung over the Oder, soaking into the lights of the lighthouse like milk into coffee. The Wrocław Mathematical Tower was dark and silent, with only the clock on the façade counting down individual minutes, as if doing so with a personal grudge. Lena squeezed the strap of her backpack and looked back. Grodzka Street was empty, the guard's window at the university gate dimmed. Her heart was pounding, not because of the altitude, but because of the plan, which was more audacious than anything she had come up with so far. Jonah was climbing right behind her up the narrow stairs. He was carrying a metal suitcase with cables sticking out of it like plant stalks. He hissed lightly after each step, pretending that the carrying was a titanic effort. - Tell me again why we risk crossing the astronomy circle off the list? - he muttered. - 'Because officially the entrance is closed for renovation, and unofficially,' Lena replied without turning her head, 'it's the only place in the city where the meridian is marked out in stone, not virtually. Our module needs a real point of reference. - 'Our module,' he repeated with exaggerated tenderness, tapping his finger on the case. - In the vegetable lunchbox. Romantically. If it ends up in a museum one day, the curator will describe it as 'the first Len-Jonas clock in an ecological package'. She smiled, though her fingers were icy. It all started when they managed to catch an anomaly in the school's workshop - the short, unsteady jerk of the rubidium module's seconds hand when they managed to synchronise it with a radio signal applied to the metal meridian line on the old city plan. The effect was so subtle that any professor would have waved his hand. Except that the jerk kept coming back. Always near the old tower. At the last platform, Lena stopped and took a flat key out of her pocket. She had learnt its shape by heart - the service door to the terrace opened hard, metal against metal, as if in a creaking casket. She shuddered as the lock rasped. The air in the tower was thick with dust and old wood. In the semi-darkness a long brass band was drawn - a meridian marking drawn across the floor like a scar. - 'I can't believe we're here,' whispered Jonah, and for a moment he was truly silent. - It looks like the inside of a watch from the inside, only magnified. There was also a forlorn refractor standing in the room on a tripod, the lenses dull from dirt, a sky map with bent corners lying next to it. On the wall hung a clock mechanism with a weight in the form of a lead ball. Each 'tick' sounded as distinct as if someone was slicing the air with a knife. - 'Zero Wi-Fi, zero GPS,' Jonah recounted, placing the case on a stone. - Just our rubidium, your courage and hundreds of years of tradition. What do we do? Lena knelt by the meridian. She ran her finger along the brass line. It was cool and smooth, like an ice rink. From her suitcase, she took out a silvery cube with an e-ink display. Nanoseconds counted down on the screen, almost silently. - 'First the anchor,' she said, connecting the clip to the metal strip. - Then the pulse. If the differences build up again, we'll try to amplify them. - 'Interpret it as if you were talking about music, please,' Jonah asked. - I don't want to think about the build-up of differences when I'm legally illegally in the building. - It's like pizzicato instead of legato - smiled Lena briefly. - Instead of flowing smoothly, time gets a jolt. The question: will she respond furioso or just sigh. The darkness outside was thickening. From the terrace, the insistent squawking of seagulls and the distant braking of the tram could be heard. The tower itself seemed to breathe, as if it had lungs made of bricks. Lena glanced at her wristwatch. 23:59. - Are you ready? - She asked. - 'I've never been more ready and more terrified at the same time,' said Jonah, grunting and leaning over to press the metal button on the device. - Three. Two. One. An impulse, short as the touch of cold water, passed through the meridian. For a split second, nothing happened. Then Lena felt the sounds in the room diverge - as if one layer of 'tic' was minimally late to the other. Her skin became covered in goosebumps. On the cube screen, the numbers began to jump, no longer in an even rhythm, but with a whimsical acceleration. - Can you see it? - breathed out Jonah. - And this is just the introduction. The brass line brightened. It did not blaze or sparkle; the light resembled the glow of plankton on the sea - milky, soft, out of this world. At the same moment, the weight of the clock vibrated and lifted a centimetre, as if someone had pulled an invisible string. The air narrowed and the familiar tone of the time signal buzzed in Lena's ears, only lower, rougher. - Lena... - Jonah pointed to the glass on the terrace side. - See. On the dusty glass, on the inside, someone with a trembling hand had scratched out the letters. They were fresh, as if the scratch had just pushed the dust away. Lena walked over, applied the torch from her phone. The inscription read: "Lena, do not open". Underneath - the date: 18/10/1883. She didn't feel the phone slip from her hand and land softly on the carpet, because there had never been a carpet here. How did she remember that twist of letters, that teething at the 'L', that tiny error in the number eight? Because it was her own handwriting, like a reflection in a mirror, only from another time. - 'Someone's making a joke,' Jonah muttered, but he sounded like someone laughing in the dark so as not to hear his own fear. - 'Or your future self has access to a very old glass. Lena returned to the meridian. The light was not fading. Yes, it pulsed, as if the floor was breathing. Each flash corresponded to a sound from the mechanism. She also heard something else: the clatter of hooves on the cobblestones, absent in today's city, the distant ringing of a tram long gone. The smell, too, had changed - alongside the dust, there was a note of lamp oil and damp brick, the kind that had never seen electricity. - 'If it works, it works too well,' said Jonah quietly. - Let's pause. Before... - Before what? - She asked reflexively, but her voice stuck in her throat. Something rustled under their feet. Through the glaze of the floor, Lena could see the outlines of a lower floor - a storeroom or former archive. Only instead of modern crates, wooden trunks with metal fittings stood there, and a tiny light flashed between them. The sound flowed differently now, as if someone had cut the ribbon and fastened it with a new buckle. The weight of the clock jerked and fell; the chain rang with a short aria of steel. The hinges in the tracery in the middle of the room, which they had never noticed before, creaked and twitched as if someone were undermining them with a chisel from underneath. - There's a... hatch here? - Jonah was already kneeling by the edge, his fingers searching for a handle that wasn't there. - It had never been in the plans. - 'Don't open it,' Lena reminded him mechanically, but at the same time she could feel her finger muscles trembling with curiosity. She was not a person who retreats. She was a person who wants to see, to understand, to name. This was what both notes and equations had taught her. A quiet clatter came from below. One, two, three - regular as a metronome. Then something pressed against the underside of the trapdoor and sprang up, as if checking to see if anyone was there. Lena and Jonah looked at each other. In the light from the meridian, his eyes looked almost silvery. - 'That can't be true,' Jonah said and bit his tongue, as if to disenchant his own words. Lena leaned lower, smelling iron and some ancient smoke. Then the top hand of the clock stopped mid-second and moved back two teeth. From below it answered - not the sound of a tool, not a creak, but a whisper. A single word, spoken in impeccable Polish, barely louder than a thought: - Leno. Before she had time to move her hand away, the trapdoor visibly moved under her skin, as if someone was lifting it from underneath. The light on the meridian blossomed in one broad pulse, and all the sounds in the tower merged into one absolutely even tone. Something heavy rubbed against the stone on the other side of the hatch, and a wooden bolt that shouldn't have been there began to move slowly, inexorably to the side.