The rain came without warning, blurring the outlines of the neon signs and pasting wet sheets of light onto the cobbles. The Vistula breathed heavily, as if someone had just cranked up its pulse; the current whipped against the pillars of the bridge, carrying with it branches, leaves and debris that flashed once and disappeared, drawn into the black ribbon. Lena stopped just beyond the fifth pillar, a little in the shadows where the calls of the night runners rarely came, and the drops from the edge of the span fell in an even, insistent rhythm. On her phone she still had an email open from five years ago. Sender: L. Meller. Subject: "The fifth pillar". Content three sentences long, dry, without poetics, like everything with the professor: "If you can still hear the old purr of the river, come. After midnight. I'll show you the plan we're not allowed to write about." Then silence, and a day later - the news on the radio that the professor had disappeared from the flat, the door open, the espresso undrinkable, the wallet on the table. For a long time Lena didn't dare open that email. And then it was too late, she told herself. Or so she thought. Now, in the compressed pocket of her jacket, she was warmed by the weight of something that could not be dismantled or ignored: a key of patinated bronze, found in the lining of an old river atlas during a clean-up in the Cartography Museum's storeroom. On the shaft of the key, a bas-relief resembling a stream rippled; on the tooth she had once spotted an engraved, barely visible '0'. Victor, the night janitor, only smirked. "You don't carry random keys to the right place by accident," he stated philosophically and offered a torch and his company. She was supposed to refuse. For some reason, she didn't refuse. - 'It's getting cold from all that water,' Victor now muttered, adjusting his hood. - And do you hear that? It's not the rain. It's like... bells. Lena had picked them up earlier, too - quiet, as if someone had moved the thin glass in the guts of the bridge. It could have been a draught between the pillars. It could have been a tram on the other side. It could have been, but it wasn't. - Remember what you said about the wall? - She asked, putting the torch to the concrete, just off the ground. - 'That every bridge has a place that likes to crack,' he replied. - And that this one was always cracking here. Up close, the wall was not ordinary. Beneath a layer of three different spray paints and dried smudges was something else - a line that was not a crack, rather an old grout, too even, too consistent. At one point at knee height, the concrete stood out slightly, as if someone had once pressed a clamp here and then forgotten they had done so. - A torch - Lena asked. She moved the light slowly, like reading a map of a foreign land, piece by piece. And then she saw - a tiny irregularity, a round splinter the size of a pea, where the concrete was darker, and in the middle, under a thin film of rust - a shudder went round her - something metallic shone, a shape almost matching the tip of a key. - It's not a wall,' she said quietly. - It's a door. Victor cmoched a second time, this time with appreciation. - Sometimes a city is made up of doors and those that pretend to be walls. I always preferred the former. Lena rubbed her palms against her trousers. Her skin tasted of salt and copper as she reflexively licked her lips, her eyes painful from concentration. She inserted the key into the navel of the concrete. It fit. Not the way keys fit in modern locks - with the precision of a factory - but the way you fit your hands into old railings: through the memory of the material. She turned it. Nothing at first. Then something vibrated, so deeply that she felt it in her bones. A street lamp a dozen metres away dimmed, as if someone had cut off the current's breath for a moment. The water sighed higher. - 'Oho,' said Victor, as if he had just heard a familiar refrain. - It's still holding, the foul beast. The lock, if it was a lock, darkened, drank the torchlight and gave it back in a shade of greenish sheen. Lena felt the key heat up between her fingers, but it didn't steam - rather like a stone taken out of a river at dawn. The smell was suddenly strangely familiar: the dust of old books, the dampness of cellars, something sweet, like lilac syrup, and sharp, like freshly cut sticky tape. She fumbled in her pocket for her phone; the display flared for a second and then went off by itself, even though the battery was full. - 'Step back a bit,' whispered Victor, and moved to shield her with his body, as if that would make any sense if the concrete decided to reach out for them. Imperceptibly, the rain stopped. The drops that had fallen from the span suddenly turned into an ash of light and, before they touched the ground, blurred into a mist gathering at their feet. From within the concrete came a sound that sounded like nothing on the surface - not a creak, not the typical metallic clack, but a high, clear tone, like when a finger nudges the rim of a crystal glass. - 'Meller once wrote about the Zero Number Passage,' Lena snapped out. - About the ground floor of the city, which is not on any floor. We laughed about it at a seminar. - Nobody likes to be laughed at - murmured Wiktor. - Even places. The door bounced off the door frame to the thickness of a fingernail. The hook of the concrete, the one that pretended to be a bracket, rose and fell; Lena slid her fingers over the edge. A cold chill hit her wrist until the hairs on her skin stood up. The hole smelled at night in a large station in a foreign city, where trains arrive unannounced, and a little like the way her grandmother's cellars in Biała Podlaska smelled, where jars of gooseberries were kept. The two of them levered - she with a spanner, he with his arm - and the concrete gave way another centimetre, two, three. A strip of light slipped into the gap, as if someone on the other side was holding a lamp. At first Lena thought it was her torch reflecting strangely, but no: the light was warm, yellowish, moving, as if it were breathing. And there was something else: the echo of footsteps, small, like the tapping of a suitcase wheel on stones. - Can you hear it? - Lena asked, although the answer was obvious. - I hear it - Victor moistened his lips. - And I don't like it. Before they could push on, the concrete did the rest itself, pulling apart with a quiet sigh. Beyond the threshold a staircase began, narrow, running down an arc like the inside of a shell. The stone was smooth and damp, and in the crevices between the steps shone a pale greenish light that Lena recognised - without knowing from where - as that of plants, what feeds on the river. The darkness here was not black, but thick, like a fabric that you can slip off with your fingers once you know where to pull. - 'Strange,' said Victor. - I can smell the field flowers. And bearing grease. Downstairs something rang out, a single tone that suddenly merged into a short, four-minute signal, fixed in the memory of anyone who has ever missed a train. The signal of departure. Lena felt a cold shudder enter her ice ladder, and she already knew that nothing that would happen now could be described later in the survey or report that her museum was so fond of. - May I? - She asked, as if anyone but herself and Victor were bound by courtesy here. - You could, since you heard the sound,' he replied. - 'But you know, Lena, not all lights lead home. She smiled half-heartedly - it wasn't a day for good advice. She rested her hands on the cool stone on either side of the walkway and took a step forward. With each foot she put down, she felt the pressure of the air change, as if she were descending into deep water, yet nothing nudged her ears. On the fifth step, maybe the sixth, she heard her name. - Leno. Someone said it the way the professor did, unhurriedly, as if each 'e' and 'o' had its due length. The voice did not echo - he was there. Victor moved nervously at the top of the stairs, but did not come up behind her. It was hers. He didn't say it, but she knew. - Professor? - she called out and, contrary to expectation, her voice didn't shatter into pieces but slid down the handrail. The light twitched. A smudge emerged from the gloom - not so much the shape of a figure, but the outline of a lamp, old, cast-iron, with a heart of embers instead of a bulb. For a second she saw a section of the wall to her left: a blackboard with convex letters, clad in a green patina. The letters folded into a word she didn't know, though it sounded familiar and foreign at the same time. Grooves flashed between them, like the marks left by the rays carved into the stone over the years by someone who waited and counted time. From below, something heavier rumbled than should have been possible in such a narrow passage. It wasn't thunder - the sky had fallen silent - it was the clatter of distant wheels on what might have been a track, if tracks can last to the depths where another's geography begins. The air rustled, like old pages turned over in an airing, and the cold light from the plants turned golden, as if joined by something alive and warm. Lena turned her head upwards. Victor was now just a silhouette against the wide, wet world. He nodded to her, more with his eyebrows than his hand. That much was enough. She took another step, slipping into the glow of the lamp, which trembled like a tame animal sensing touch. - 'Leno,' repeated the voice, closer this time, just beyond the first flight of stairs. - 'You're only five years late. We still have a moment. A hand emerged from the darkness, thin, with long fingers that once moved across maps with reverent care. There was a glimmer of metal on its wrist, not a watch, but a ring with a fine scale, as if someone was carrying a time measure. The hand held a lantern. Behind it, something murmured, like a giant pet, and another tone rolled - the same as at the stations, only stripped of the loudspeaker, more real. Lena reached out before she had time to think sensibly. Her fingers almost touched the cool metal of the lamppost as the steps beneath her feet vibrated, and the spelled letters on the wall, right next to her face, lit up for a moment, forming a new inscription that hadn't been there a second ago. The current of the river slammed against the pillar with a force that made the bridge groan. Something was coming from below, bigger than reason allowed, and she could already feel on her lips the taste of the wind that always precedes the arrival of a train from another direction, when lights flashed from behind the arch and....