Through the door under the clock

The Maria Skłodowska-Curie School looked like any other, but Zuzia, Kuba and Lena knew it held a lot of secrets. One Friday afternoon, when most of the students had long since left the corridors, they were still hanging around the biology room. Just then, Lena found a key with a strange green stone under the mop cupboard. - Maybe it's for some old locker? - wondered Kuba, looking at the key under the light. Susie, however, noticed something else. At the end of the corridor, right next to the big old clock, there was a thin line outlined on the wall, almost invisible. They walked closer and discovered a door that no one had seen before. It was narrow, made of dark wood, and a shade of green glinted in its lock - exactly like the one on the key. With excitement, Lena slid the key into the lock. They heard a quiet click and the door swung open of its own accord. On the other side was a winding staircase leading downwards, bathed in a slightly phosphorescent light. - Ready? - whispered Susie, and her voice echoed. Carefully they began to descend. The walls were covered with paintings of unknown animals and strange symbols. The lower they descended, the more the air smelled of something sweet and fresh, completely unlike an ordinary cellar. Suddenly, the stairs came to an end. In front of them they saw a huge blue cave in which colourful islands floated and flying fish flew. Everything shone in the glow of a strange golden mist. On the other side of the cave, on one of the islands, stood a creature resembling a huge owl in a cloak. It had a huge key in one paw and was looking straight at the children. Suddenly the island with the creature began to move towards them....