The riddle of the underground portal

When Maks and Hania found out that they were going to spend their entire holiday at Grandpa Antoni's house in the countryside, they were convinced they were going to die of boredom. Grandpa was nice, but he lived in an old guesthouse at the end of the world. There was no internet or even decent coverage. Maks liked his computer games and couldn't imagine weeks without his phone, while Hania would be happiest lying on her bed with a book. However, on the very first evening, Grandpa warned them not to go down to the cellar. He said this quite seriously, with a glint in his eye that only fuelled their curiosity. - The basement is no place to play. 'Promise you won't go down there,' said the grandfather, pulling a huge, old-fashioned key from his pocket and hiding it in an old tea tin. But as children do, Maks and Hania could think of nothing else. The next day, when Grandpa was snoozing in his armchair and the house was perfectly quiet, they went into action. In the kitchen, they quickly found the tin and in it the key. Hania's heart was pounding like mad as she held it in her hand. - Are you sure we should? - Maks asked, but it didn't sound like a real objection. - 'Of course,' she replied, already heading for the cellar stairs. The cellar was cold and smelled of dampness and old apples. They stepped carefully up the creaking stairs, looking around in the semi-darkness. At the very end of the long corridor they spotted a door - different from any they had ever seen. It was tall, made of dark wood, and had strange symbols carved on its surface that resembled mysterious letters. Maks inserted the key into the lock, which fitted perfectly. Their hearts beat faster as he slowly turned the key. The door opened with a quiet creak and.... Behind them was no ordinary room. Instead, a blue portal swirled in the wall, swirling and shining like a sheet of water that shouldn't exist in this place. - Is this... - began Hania, but did not finish, because suddenly a strange sound came from behind the portal: the flutter of wings and a quiet, melodious laughter. They stood right in front of the portal, staring into the swirling abyss. Would they enter? Will they discover what hides on the other side? And then something - or someone - pulled a bright hand out of the portal....