It was the end of June, and the sun was spilling wide over the small town of Sobótka. The summer heat was creeping into the houses, scratching at the windows and forcing the inhabitants to hide in the shade. However, Maja, Oskar, Nina and Bartek were not the kind of people who liked boredom. Instead of another day spent in front of the phone, they dreamt of something special. It was Maja who was the first to make a suggestion: "Have you ever heard of Green Canyon?" she asked, sitting down on the grass in the park. "Apparently nobody goes there because it's a former mine. But they say you can find amazing things there if you dare." Oskar just squinted. "You'll probably only find old bottles and rusty screws there. But why not? Sounds better than sitting here all summer." Bartek, who always carried a torch and pocket knife in his backpack, was already planning the route on the map on his phone. "It's about two hours by bike through the forest. It's important to be prepared. Take water, something to eat and trekking poles. You never know what's waiting at the bottom of the canyon." Nina, the youngest of the group, was excited, although a little scared. "What if my parents find out? And if we get lost among the trees?" Maja smiled mysteriously. "That's exactly the point. Let's experience something we won't forget for the rest of the holiday." That's how, later that day, the four of them met in front of the grocery shop, each with a small backpack. Hoping for an incredible adventure, they set off through winding forest paths. They passed tall, lush ferns, dashed through shade-sunken glades, until the air felt heavier and the trees grew thicker and taller. After two hours of strenuous pedalling and a few stops, they finally reached the edge of the Green Canyon. Ahead of them, the ground suddenly gave way, revealing steep walls overgrown with ivy and moss. Strange geometric shapes loomed below - like the remains of old machines or walls from another world. Oskar was the first to get off his bike. "Who's first down?" he chuckled cheekily, but there was a note of uncertainty in his voice. The canyon was unusual. You could feel a slight thrill, as if the place had been waiting for years for someone to dare to descend to the very bottom. With the help of Bartek, who unrolled a rope and checked that the ground beneath their feet was stable, they began their cautious descent. Stepping over boulders, they felt the crunch of gravel under their feet and heard the echo of their own footsteps reverberating off the rock walls. At one point, at the level of a rock shelf, Nina noticed something unusual. "Look over there, behind that big rock!" she pointed with a trembling hand. Something shiny flashed through the gap. The four came closer, carefully sliding down the loose pebbles. Behind the stone was an old steel door, almost completely overgrown with ivy and moss. A door that definitely didn't belong in an abandoned canyon. Bartek pushed back some plants and discovered a knob, rusty but still functional. "Do you think we should open this?" asked Nina, clutching her torch. Oskar was already pulling the knob, however, and the rusty hinges groaned quietly. From inside the door came a cold breeze and a darkness so deep they felt shivers down their spines. At that moment, something inside moved and an indistinct sound could be heard, as if someone - or something - was waiting on the other side....