Szymon, Lena and Bartosz have known each other since childhood. In their free time, they roamed the nooks and crannies of their town, hoping to find something unusual. That day, they decided to look under the old iron bridge that stood on the outskirts of the town. The bridge was a place often bypassed by the elderly, but for the young explorers it provided the perfect setting for holiday adventures. The air under the bridge was cool, smelling of dampness and rusting metal. Bartosz, armed with a torch and a backpack full of useful items, walked in the lead. Lena was on the lookout for traces of former inhabitants of the area, while Simon documented everything with his phone camera. Suddenly, Lena noticed something unusual - a piece of rusty grating sticking out of the ground. She and the boys began to uncover it, and it was incomprehensible what appeared to their eyes: a circular entrance to a tunnel that was not on any city map. The opening was wide enough for them to crawl through. There were strange signs carved on the walls, as if someone was trying to tell something through pictures. "Do you think it's safe?" - Simon asked, looking uncertainly at the dark opening. "Maybe someone lived here a hundred years ago! Or it's a passage to the cellars!" - replied Lena with a glint in her eye. After a short deliberation, they went inside, lighting their way with Bartosz's torch. The tunnel led steeply downwards, their footsteps echoing off the wet walls. The further they walked, the more the atmosphere changed - the air became seemingly thicker, and a pale light flashed at the end of the tunnel. They stood up. In front of them stretched a huge circular portal of shiny, silvery matter that rippled like a sheet of water moved by the wind. On the periphery were the same enigmatic signs they had seen at the entrance. From inside the portal came a quiet, resonant noise, as if someone from behind the wall was whispering old legends. They took a few more steps. Lena reached out her hand to touch the silvery surface. Suddenly, the whole chamber flashed with a sharp light, and the ground beneath their feet trembled slightly....