At twelve forty-seven, as the sun reflected in the damp slabs of the boulevards and the Vistula breathed a slow wave, all the clocks in Warsaw froze. Trams slowed to an almost motionless glide, lifts stopped between floors, delivery drones hovered over the avenues like paper ravens. For four minutes, the city sounded only the background - the distant siren, the clatter at the bus stop, the breath of the crowd. Leon Krawiec sat in the glass-walled room of the Rhythm Centre on Tamka Street and stared at a wall covered with a fan of seconds - a screen on which each pixel was a separate clock: atomic, satellite, quartz, biological. They all stood still like blades of grass under ice. His own watch on his wrist counted down to nothing. It displayed a fixed time, as if time had turned to stone. - 'This is not a local emergency,' said Marta Jaskiewicz, running in and shaking raindrops from her hair. She was wearing a jacket whose sleeve was saturated with sensors. - The reference had been cut off. It must be coming from downstairs or... from somewhere no one looks anymore. - Satellites? - Leon raised an eyebrow. - The satellites are clear. The longwave channel is silent. I pulled up the logs. Look. She handed him a slim disc of records. A sudden gust blew across the charts like strands of grain: a perfectly even pause, four minutes, zero jitter, zero random oscillations. It was as if someone had laid a hand on the city and pressed pause. - Four minutes of silence,' murmured Leon. - Who would have access to such a layer? Martha smiled crookedly. - The base node. The one the plans say we switched off five years ago. Under the Palace. Leon blinked. He remembered the case: moving to a new system, more redundancy, duplicated atomic time standards located in secure shelters outside the city. The old ground node, still built when no one trusted orbit, was to be disconnected, sealed and turned into a museum for students. - 'Paper will accept anything,' Marta added. - But the cable can remember. Are you coming in with me? They drove down two hours later, when the traffic returned and the city began to twinkle in its rhythm again. The pass Marta pulled opened a metal door in the lowest level of the Palace. The corridor smelled of the clatter of old lifts and dust. The light in the tunnel was too blue to be from the era the plaques spoke of. Data strips stretched along the walls: yellow cables, grooves of fibre optics, new muffs plugged into old armour. - 'I had a class here in my first year,' Marta said, touching the cool brick with her fingers. - They taught us that the clock is infrastructure like water. Like oxygen. You don't notice it until someone turns it off. - 'Today someone turned it off,' replied Leon, sweeping the ceiling with the light from his torch. - Four minutes. Precision like a surgeon. They stopped in front of a heavy door with a knob that looked like a relic of a bunker. A small LED glowed above them. It pulsed slowly, insistently. Leon felt his own pulse begin to tune into that dot, like an alien metronome. - 'It's here,' Martha said. - Node Zero. - Why Zero? - Because this is where you used to zero all the city clocks after tests. Reference point. The beginning of the scale. Leon leaned over to the panel. Above a layer of dirt, two porcelain-framed touch panels were visible, so archaic as to be new. The inscription read: "Please report for tune-up". Underneath glowed a caption about the node's parameters, updated yesterday. - Yesterday? - Leon looked at Marta. - 'Who was coming down here? - The logins disappeared at two o'clock in the morning - she replied. - It was as if the corridor had swallowed its own record. Leon put his hand on the panel. A cold went through his skin. The screen hummed and unfurled a string of characters: clean, even typographically beautiful letters, with soft tails, as if someone had designed them for timeless readability. "Recognised: LEON KRAFT. Resource: urban CHRONOMETRY. Status: active." Right next to it, after being touched by Martha, a second line appeared. "Recognised: MARTA JASKIEWICZ. Asset: ADAPTIVE INFRASTRUCTURE. Status: active." The light above the door glowed white. The knob unlocked with a soft clatter. Leon looked at Marta. She had that look that meant: we can't back out. He rubbed the back of his neck with his fingers, as if he could wipe away the tension. - 'Before we go in,' he said, 'tell me what the logs you didn't show me tell you. Martha sighed. - That the signal of silence came from here. That it was perfect and that it carried a headline, a signature of sorts: "The Zero Age". Without a digital signature, but with something else. Sometimes composed of the geometry of the city. - Of geometry? - You know how you measure length on a map, you convert it into a road. They... whatever it is... have converted the distribution of stops, the distances between points, the layout of bridges, the angle of light on elevations. The signature of Warsaw encoded in nodes. As if the city were a clock. Leon felt the remnants of coffee in his stomach solidify. - Or as if someone had learned to count time by the shadows of our buildings. - Exactly. They lifted the knob. A door clanged against the wall, revealing a narrow vestibule, followed by another room. Inside there was a chill, metallic and dry. In the middle stood a low pedestal with a transparent dome. In the dome, something pulsed - regularly, burning and extinguishing with a selective glow, like the heart of a jellyfish. All around - wreaths of wires, as even as a technical drawing. On the left wall hung a screen that did not emit ordinary light, but a strange mat - as if the brightness adhered to it in a thin layer. A map of the city in black and milky white appeared on it. The Vistula ran without error. Above it grew a web of lines that were not roads, not tracks. They were intersected by red dots. Each dot flashed four times, stopped and flashed on. Then it all merged into a single pulse. - This is... - Marta began. - ...a frame of reference - finished Leon. - But made of the city itself. Then his watch vibrated. The display jumped back four minutes, as if a spring had suddenly let go. A soft sound rang out, longer than the designer would have allowed. A light in the vestibule blinked. The screen on the wall showed a new block of letters. "Please apply correction: -00:04:00 Reference source: Epoch Zero. Reference: Vistula, noon, day 13." - Day thirteen of what? - whispered Marta. - I don't know. - Leon felt the words sound in his mouth like someone else's. They moved closer to the dome. The material was cold and smooth. In the centre lay a disc, flat and disturbingly straight: dark matter that did not reflect light like ordinary metal, but absorbed it right up to the edge. On the rim - small signs, incompatible with any alphabet Leon knew. And he knew several. They were arranged in a rhythm that resembled a beat, but not a heartbeat. More like a step measuring the distance between shadows. - 'This is not ours,' Martha said, and there was no pathos in those words. There was fact. Leon brought his hand closer until he could feel above the dome a chill that couldn't be coming out of the matter, but was coming out nonetheless. His finger hovered over the weakest part of the case, over the thin border that suggested a lock. Someone had designed it so that it could be - so that someone would dare - lifted. - 'Stop,' Martha said, capturing his wrist. - 'Reading first. If it's a reference generator, it mustn't be unsettled. - Or if it's not a generator at all,' Leon muttered, but withdrew his hand. - 'Check the ports. Her hands danced at the console. The connectors woke up like an old animal, reluctantly but with recognition of touch. A new image slid out on the screen: lines of code, so simplistic as to be beautiful. A meta-description that used the poetry of topography instead of network addresses. At the very end was a line that read: "Entry authorised. Waiting: two people. Initialisation of dialogue after audio notification. Password: first beat in new rhythm." - Sound password? - repeated Leon. - How are we supposed to know a sequence we don't know? Martha stepped back and looked at his watch. - Does he have a microphone? - He does. - Leon moved his wrist. - Just why... The warmth in the room changed slightly, as if someone on the other side of the air was holding their breath. The LED above the door stopped flashing and went solid. The outer corridor spat out a brief wave of chill. The screen blinked and for a second, literally for one blink, it displayed an old photograph: the smile of post-war Warsaw, the smooth rails of the trams, the sun on the wet cobblestones. Then the map came back. - 'Did you see that? - Marta drew in a breath. - I saw it. It was... - Leon swallowed. - Whatever it is, it can display the past. Or something that was it. The sky above them responded with distant thunder. Probably a thunderstorm from the direction of Prague. But the sound reverberated strangely, as if someone had fashioned a new echo in the sewers beneath the city. The dome flared again. A quiet tone flowed from the console, knotty, steady, perfectly even. A tone that resembled the signal of history played back from end to beginning. - 'That's it,' whispered Marta. - The first beat. - If we repeat them... - Then we'll let what waits behind the door into our clock. - And if we don't repeat it, the city will stand again. They suspended their gaze on the dome. For a fraction of a second, it seemed to Leon that a shadow moved beneath the surface of something dark - not the reflection of their faces, but another image, as if someone was standing on the other side of the glass waiting to speak. His hands trembled, not from fear, but from tension, from the fact that something was about to shift and the world would be a hair's breadth away from a different rhythm. The tone froze. The LED blinked once. Only one word appeared on the screen, unsigned, written in a font so old it was new: "Please." Leon drew in a breath, raised his wrist, brought the microphone close to his mouth and....