Underground City.

Jack had wandered far from his original shelter, venturing deep into the strange world's terrain. The massive cat-like creature, whom he had taken to calling Brutus, followed loyally by his side. Days passed in exploration, and the two of them soon stumbled upon an anomaly—an entrance hidden within the side of a mountain. The rock formation was unlike anything Jack had seen before, its structure smooth and unnaturally symmetrical, as if chiseled with precision beyond human craftsmanship.

With cautious steps, Jack and Brutus entered the cavernous opening, their presence illuminated only by the dim, bluish glow of crystals lining the walls. As they ventured deeper, the air grew warmer, and a soft humming sound, akin to distant machinery, filled the space. The tunnel opened up into a vast underground city, a sight so surreal that Jack momentarily questioned his sanity.