On a Thursday after dark, Kaja slipped her skateboard off the curb and stopped by the tunnel under the station. In her backpack she carried her astronomer aunt's metal box and a heavy brass ring. On the lid someone had scratched out the phrase: Don't read the sky on Thursday, please. She left it behind her, with a letter without a signature and a sky map with a strange gap. Iwo waited by the whale wall, where the graffiti always smelled of fresh paint. Lights from the platform cut the tunnel in stripes and the dripping from the ceiling sounded like a metronome. "Are you seriously opening this?" - Iwo whispered, feigning courage, but staring at the box like it was a bomb. They were supposedly just going to check the mural and come back, but they both needed proof. Kaja unfastened the clasp and inside lay a black disc with a swivel and a thin prism. The ring clicked on her wrist, cool and heavy as wet stone. As she turned the scale, the prism fired a silent beam, drawing pale star lines across the whale. The lines glided until they were covered with veins of paint, as if the sky was trying to match the city. Kaja remembered the warning, but her aunt's whisper sounded more like an invitation than a prohibition. Her aunt had repeated that the sky was a language, and languages had to be read carefully. The air thickened and smelled of salt, even though it was four stops by train to the sea. The stains on the wall moved and the eye of the painted whale blinked as if it wasn't paint at all. The ring vibrated and beeped, and the prism played a low sound that she felt in her teeth. The clatter of a train came from the distance, but the tracks sounded as if they were turning into deeper space. The wall spread out into a vertical, shiny gap, as if someone had cut the skin of the concrete with a knife. Behind it there was no service aisle, just another tunnel, black and silent, with a vault like the night sky. The constellations had alien shapes, and from the darkness came a whisper that coined her name. "Kaja... hurry," the voice was familiar, but older, and sounded as if it knew the tunnel better than she did. Kaja lifted the disk, and the stars trembled, forming a sign above the gap that resembled her initials. Iwo grabbed her sleeve, but the wind from that side sucked at the tunnel like lungs before a scream. A silhouette in a wet cloak slid out of the darkness, and a winged, soundless thing flitted above. The ring flared to white, the prism spangled stars, and the gap began to close rapidly. "Don't go in alone," - said a voice, already at the very edge - "for there I am just waiting." More lamps went out in the tunnel, as if someone was counting backwards, and the floor vibrated underfoot.