Primary School No. 7 in Birkenau looked like something out of an illustration for an old book: brick walls, tall windows and grapevine draping around the corners like a green scarf. The best-loved spot was at the very top, behind a narrow door that always jammed a little. The attic smelled of dust, wood and summer-hot asphalt from the football pitch, and through the small windows you could see the tin roofs of the surrounding houses. It was there, under the slanted beams, that the Seven's radio station operated. It brought together pupils who liked to tell stories, look for sounds and ask questions, to which the adults often replied: "It's complicated". The loudest people here were the microphone and the imagination. Lena was eleven years old, with a freckled nose and an absolute ear for anything that creaked, rattled or hummed. She collected sounds on her phone, calling them in her own words: "the clink of a radiator", "the swish of a cross-country ski run in the corridor", "the sigh of an old wardrobe". Maks was a year older and built his own microphone from parts ordered on the internet, because, he said, no shop sells microphones "interesting enough". While Lena did the talking, he kept an eye on the knobs, cables and all those flashing dots on the screen. On that September afternoon, the sky was streaked with pencil-coloured clouds. Art circle and basketball practice were still going on in the downstairs rooms, so the school hadn't gone completely quiet, but the attic was almost empty. Lena was setting up the recording rack and Maks was untangling cables. In the corner, covered with a cloth, stood a small, greenish bell. It was called the Bell of Silence because, according to a faded plaque, it had not sounded once in decades. It used to belong to the vestibule, but after renovation it ended up here: seemingly a keepsake, seemingly a piece of furniture. - 'Today we're doing an episode about school noises,' Lena decided, putting on her earmuffs. - We'll show that even silence can be loud. - And that my microphone can even hear blinking,' replied Maks proudly. - It's that new noise filter, see. A line was moving on the laptop screen, which vibrated with every step on the wooden floor. Suddenly there was a draughty, deep sound, so clear that the air seemed to tremble. Lena felt it in her stomach, and Maks's headphones on his ears popped up. For a second, nothing crackled, scraped or rustled - just that one tone, as round as a packet of candy canes. - Did you hear that? - whispered Maks, as if he could startle the sound. - I heard... - she replied, taking off her ear muffs. - But from where? They turned around simultaneously. The bell under the canvas had not gone far. It stood as it stood, completely still, with no string, no heart, just a thin net of cobwebs around the edge. The shadow of a cloud passed through the window. A slight gust moved a rolled-up map standing in a tube next to the technical director's desk. The tube, which no one was usually interested in, suddenly twisted and - unbeknownst to him - the map slid out with a soft rustling sound, unrolling a wide sheet on the floor. Cardboard held it by a corner, as if on purpose to prevent it from rolling back up. - 'An old map of the school,' muttered Maks. - 'Do you see this stamp? 'SP No. 7, year 1963. Gosh, there's even a drawing of our biology room, just.... as if in a different place. Lena crouches down. On the floor plan, between the history room and the music studio, a narrow corridor was drawn. Completely incongruous, because in fact a wall with a display case full of diplomas ended there. The corridor on the map had a designation something like 'K-3A', written in pencil, then erased and redrawn again with a thinner line. - 'There's no passage there,' Lena said, although it sounded more like a question. - Unless there is, we just don't know about it," replied Maks, hanging his head for a moment, looking at the bell. - And that we were just invited to it? A short rumble of balls bouncing on the dance floor came from downstairs, then silence. In the art room someone played the radio for a moment. The steel ladders in the gymnasium creaked, as if repeating the same sound of the bell, only in a clumsy, metal way. - 'We're taking the microphone,' Lena decided. - And a torch. And maybe... you know, something to take notes on. Maks was already packing the equipment into his backpack. He also picked up the unfolding map and rolled it up carefully, like a roll of dough. As they walked past the bell, Lena lifted the canvas and peeked underneath. The edge of the metal had smooth, worn ornaments and a small engraving of an oak leaf with an acorn. - 'Like our school emblem,' she said quietly. - The same leaf. - On the emblem the acorn is on the right, and here it's on the left - replied Maks automatically, who had an eye for detail. - But actually, similar. The stairwell was in semi-darkness, like when the day can't decide whether it should be evening already. Neon fluorescent lights dangled from the ceiling, blinking lazily as if they too had their breaks. Footsteps echoed. As they passed the first floor, they looked into the library. Mrs Rose was just closing the borrowing windows. - Ms Róża, did the layout of the corridors in our school used to be different? - Lena asked, trying to sound like she was conducting a serious interview. - Everything used to be different - smiled the librarian. - But this building is unlikely to have changed spectacularly. Why do you ask? - Because... Because we are doing an episode about the history of the school - interjected Maks quickly. - And we're curious about what's disappeared and what's left. - The bars in the canteen are gone, the wooden pencil cases from the lending library are gone, the holes in the floor are gone.... and what's left is what should be," replied Mrs Rose, looking at them calmly. - And elbows to each other on the stairs, please. And no racing. - Sure - they said at the same time and moved on. The third floor smelled of chalk and floor paint. The diploma cabinet shone as bright as ever, full of colourful cards that remembered old recitation competitions and chess tournaments. On the left was the door to the music, on the right to the history room, where someone had drawn a Viking on the chalkboard. In between - a wall. On the map there was a corridor. Here was the wall, smooth, painted a warm beige. - Well, that's nice - muttered Maks. - Maybe the map was just some sketch gone wrong. Lena came closer. She touched the paint with her hand. Cold, hard, ordinary. And yet... The bulge just above the skirting board was unnatural, as if there was something narrower than brick under the beige. Near the floor, in the corner, the wood had a tiny shape cut out of it. Until she froze. - Look," she said. - Acorn. In the corner of the slat someone had carved a tiny acorn with a leaf. It was so thinly made that it must have escaped most eyes. But once you spotted it, it was impossible not to see it. - 'Don't touch it,' whispered Maks. - It will still set off an alarm or something. - What alarm? After all, it's just carving. She pushed the little acorn with her fingernail. With a set of cables dangling from her arm, it looked serious, though in truth her heart leapt up into her throat. Nothing had happened. No sirens, no lights. Only a sort of quiet, deep breathing came from inside the wall. A scratch, a click. - Did you hear that? - She asked, and Maks only nodded. He brought his ear closer to the wall. He touched the slat. He moved his fingers along the bulge. At one point his hand trembled, as if he felt a break in the touch. - Here," he said. - There is a gap here. Lena took a thin pencil out of her pocket and slid it into the crack. It went in. The beige paint tightened. Something crunched quietly, as if a tape that no one should have touched was peeling off. A speck of dust fell from the ceiling. A warm draught brushed their ankles. - 'We don't have a permit,' Maks remarked reasonably, but there was a tremor of fascination in his voice. - We could flop. - We can also record something that no one has heard before - replied Lena, switching on the microphone. A green LED lit up like a cat's eye in the darkness. The fluorescent light above the display case flicked three times. Silence took a deep breath, and then - even before they had time to call it - a soft tapping sound came from behind the wall. Not like hitting a wall. Rather like fingers inquiringly knocking on a table. One, two, three. A short pause. One, two. - Is this some kind of cipher? - muttered Maks. - 'Or an echo, or...' - ' broke off Lena, because at that moment the gap at the slat widened by a millimetre and something bright flashed between the beige and the shadow. Not the light of a lamp. More of a ray that had no right to escape from where there should have been only a brick. Somewhere far away, the coach from the auditorium spoke: a whistle, a shout, the clank of a hoop. Up here on the floor, the world had turned down to a whisper. Lena felt the hairs on her arms grow stiff, like a cat listening in. Maks squeezed the shoulder straps of his backpack so tightly that his knuckles turned white. - One more millimetre," he wheezed. - Just a little more and... Do you hear that? On the other side of the wall, the floor creaked. Someone - or something that could walk - moved slowly, stopped right next to the invisible door and, almost completely silently, nudged the answer: two shorter knocks, as if counting along with them. Lena raised her hand to the acorn on the slat. Her heart was pounding in such a way that she feared the recording would be destroyed by the rhythm. They looked at each other. Neither of them blinked. The draught from inside the wall smeared their knuckles again, this time warmer, like from a door that swung open eagerly. - Ready? - she whispered. - Three... two... - answered Maks and put his fingers on the edges where the beige was transitioning into shadow. - One...