The historic Ignacy Sikorski High School always had a slightly stuffy atmosphere - high ceilings, creaky staircases and whispered gossip about events from years gone by. For Natalia, Bartek and Lena, it was their last year before matriculation, so they usually just passed these oldies around, thinking about exams and proms rather than the building's past. Everything changed one November afternoon when their physics teacher, Mrs Szymańska, suggested they take part in a science competition. - I could use a topic with a "wow" effect," Lena muttered, flipping through the pages in the library. - So many interesting topics, and everything has already been done," sighed Bartek. Natalia raised her eyes from the book. - How about we investigate something on the spot? The school has been standing here for a hundred years, maybe we can find the remains of old experiments? They came up with the idea of searching the school archives and the basement. The headmaster agreed reluctantly, handing them a large, heavy key. - But only to the areas marked with green tape. The rest was renovation stuff, better not to go in. They descended into the basement in the early evening, the telephone lightening the darkness. The rows of old cupboards, the smell of dust and the occasional sound of dripping water all made them feel like explorers. At one point, Lena stumbled over a bulging tile. She stopped and looked under her feet. - 'Oi, there's something here,' she whispered, bending down. Bartek approached, shining his torch harder. They moved the tile away - underneath was a narrow passage surrounded by bricks. Not even the green tape was around. - This looks like... a new entrance? - Bartek asked. When they blew into the opening, they felt a breath of cool air. They entered, first Natalia, then Lena, and finally Bartek. The corridor turned out to be surprisingly long and led deeper into the building at a slightly sloping angle, as if descending deeper. At one point they came to a metal door with the school emblem stamped on it long ago, barely visible. The door was slightly ajar, and there was a strange, quiet humming coming from behind it - as if some kind of machine was working there. - What's in here? - whispered Lena, exchanging a glance with the others. Suddenly, a faint, pulsating light flashed inside. Holding their breath, they pushed open the door. And then...