If someone had told Jagoda a fortnight ago that one sleepless night she would become some kind of urban explorer, she would have laughed out loud. She did not feel herself to be a person of extraordinary adventures. She preferred to read books, do jigsaw puzzles or wander around the old market square in their hometown of Glass City with Simon and Anka. But this is where it all began. It was early September and the air still smelled of summer. The town's streets with townhouses remembering the times of their great-grandparents hid narrow passages and secret alleys. It was in one of these that Jagoda, Szymon and Anka met that Saturday afternoon. Anka had a torch with her - "in case we want to go lower," she joked, showing them the entrance to the underground tunnels that supposedly stretched under the entire market square. Every child in town had heard of these passages, but few had the courage to really explore them. "Oi, are we seriously going to go in?" Simon glanced uncertainly at the rusty lattice behind which the staircase downwards began. Berry bit her lip. Although she wasn't sure if this was a good idea, her heart was beating faster with excitement. "Just for a moment, let's see what's down there." They went down one by one, the torchlight dancing on the bricks. The tunnels were damp and cool, and the echoes of footsteps reverberated off the walls. The remains of old wooden beams creaked underfoot. There was a smell in the air of earth and something else that neither of them could name. Suddenly, a pale light flashed far ahead of them. They stopped, looking at each other in disbelief. "Is someone there?" - whispered Simon. But instead of an answer, they all heard... laughter. Quiet, disturbing, as if emanating from several places at once. Anka squeezed the torch tighter. "Shall we go?" They took a few steps forward. With each metre, the tunnel seemed more and more unusual: there were old inscriptions on the walls and strange geometric drawings they had not seen before. When they reached a bend, suddenly a light flashed right next to their feet. It was some kind of small light that they had not noticed before. The friends' gazes met in silence. "This is no ordinary light bulb..." - remarked Berry, leaning over the flickering light. Next to the lamp were three metal keys, each of a different shape, and a small notebook with notes in a completely foreign language. At that moment, their laughter came again from further down the tunnel, this time clearer - and much closer than before. The friends realised that they were not alone. They wondered for a moment whether to turn back.... But then -.