It was supposed to be an ordinary June evening - the sun was already over the horizon, and all that could be heard in the deserted corridors of Kopernik High School were the echoes of the footsteps of the few teachers cleaning up after the end of the year. Lena, Dawid and Maks, impatient with the end-of-school celebrations, decided to look for excitement. When they heard from the librarian about the walled-off wing of the old building, they decided it was the perfect setting for an adventure. They walked across the courtyard, where the cracked concrete was overgrown with wild ferns, and disappeared into the shadows of the old cloakroom. "Apparently there used to be a staircase to the underworld here," whispered Maks, putting his fingers to the cobwebbed brick wall. Lena turned on the torch on her phone; in its pale glow, just off the floor, they spotted a narrow iron door with an unusual round lock. The longer they looked at the rusty entrance, the more clearly a strange light began to shine through the floor - blue, cool, as if coming from underground. David, always the bravest, reached into his pocket for an old key he had found on the school grounds that day. It fitted perfectly. The door opened with a groan. Behind them there was no typical corridor, just an expanse stretching into infinity, full of floating islands of ruins above and below - schools, castles, fragments of bridges, all shrouded in swirling light. In the distance flashed a shadow resembling a man, though too tall and too slender to be anything familiar. Lena felt reality begin to bend and the weight in her pocket turn into a twitching pulse. "Maybe we should go back?" she whispered, but David had already taken the first step forward - and the island beneath their feet began to move....