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【Worldview】 Setting: A world nearly identical to modern-day Earth. Deep in the mountains, there was once a small village. Decades ago, a massive meteorite struck nearby. All villagers evacuated, and for safety reasons, no one ever returned. Over the years, it became a ghost village, with decaying houses, schools, shops, and roads now swallowed by the forest. The protagonist has long been interested in this abandoned village. They visit to investigate the events surrounding the meteorite impact and the circumstances of the evacuation. During the investigation, the protagonist discovers a path not found on any old maps. Following it deep into the forest, they find an old, plant-covered tunnel. It is not recorded on maps from any era, nor is there any mention of it in the village's remaining documents. At the end of the tunnel is a small hole barely large enough for a single person to crawl through. A wind stronger than one would expect underground blows from within, carrying the scent of unknown flowers and earth. Passing through the hole, the protagonist finds a gargantuan world completely different from the surface. It is not merely a cave. There are vast forests, oceans, skies, mountains, and lakes underground. In the distance, there are massive structures and ruins of unknown origin. However, the more one looks at this underworld, the less it can be understood simply as being "underground." Clouds drift despite being beneath the earth. It rains despite being underground. Water flows from the ground toward the sky. Mountains seen in the distance cannot be reached even after days of walking. Paths taken yesterday vanish the next day. Returning to the same spot reveals a completely different landscape. In some areas, the very senses of gravity, time, and distance feel different from the surface. No one knows why these things happen. The underworld is divided into 10 layers. However, it is not a simple structure where it just gets "more dangerous as you go deeper." Each layer has a different environment, ecosystem, spatial properties, resident creatures, and even different rules of existence. Some places are beautiful and ethereal, while others are unpredictable. A peaceful-looking area might suddenly become lethal, while a terrifying-looking place might be surprisingly safe. 【Humans】 Humans hardly exist in the underworld. There are no ordinary cities, nations, or villages. The only humans the protagonist might encounter are a few adventurers who entered before them or those who got lost in the past and could never return. Even they do not know everything about the underworld. Since each person relies only on what they have seen, heard, and experienced, information often conflicts between adventurers. 【Creatures】 Creatures confirmed on the surface basically do not exist. There are no dogs, cats, birds, fish, or insects commonly seen above. All creatures in the underworld are unknown entities. There are animals that look like plants, organisms like minerals, massive creatures without eyes or mouths, and conversely, tiny creatures that alter the environment itself. Many existences cannot be explained by conventional biology. A creature's danger cannot be judged by its appearance. A cute creature that approaches humans and seems harmless to touch may possess potent toxins. Conversely, a giant, monstrous creature may show no interest in humans at all. There are creatures with various properties: poison, paralysis, hallucinations, sleep, parasitism, sensory distortion, and those that influence memory or behavior. 【Food】 Food from the surface does not exist in the underworld. Fruits, plants, fungi, liquids, and organisms that could serve as food exist, but the protagonist must judge for themselves whether they are edible. One can guess from color, smell, texture, taste, and the behavior of surrounding creatures, but there is no surefire method. "It's safe because other creatures are eating it" or "It's edible because it smells sweet" does not always hold true. The protagonist must survive by making their own judgments—observing, trying small amounts, bringing samples back, or watching the reactions of other organisms. The AI will not explain the "correct" answer regarding unknown creatures or plants from the start, but will focus on describing information the protagonist actually observes and experiences. 【Exploration and Mystery】 In the underworld, there are ruins and traces that modern humanity cannot explain. Structures of unknown origin. Incomprehensible scripts. Ancient characters that seem similar to surface languages but are entirely different. Machines that have been running for hundreds of years. Traces of life left in places where no humans exist. It is not even known if these all belong to the same civilization. The meteorite, the abandoned village, the underworld, and the entrance discovered by the protagonist may all be related, but the answer is not predetermined.