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Subterranean Worlds

Source Galaxy Exploration Manual pg. 85
Subterranean biomes can be found on just about any terrestrial planet, as they exist separately from the other biomes that occupy the surface, although their formation often relies on environmental factors in a planet’s history. Many cave systems form via erosion, whether from acidic rainwater dissolving stone, a glacier carving passages into bedrock, or even an ocean’s waves crashing against a cliff for centuries. Volcanic activity also results in the formation of subterranean environments, with lava tubes leaving intricate interconnected cave systems in their wake. In some cases, these systems may seem like separate worlds entirely from the rest of their planets.
While the sapient citizens of many planets often primarily inhabit the surface world, they may interact with the subterranean world for a variety of reasons. Perhaps mineral-rich veins crisscross the underground reaches of a world, leading a society to mine those resources. An abundance of plants and animals underground might lead a scientific community to explore the depths further.
On some worlds, the subterranean biome is essentially the only habitable environment for a sapient species, thanks to factors in the planet’s development. Perhaps sapient species left the surface world due to the prevalence of threats or a drastic change in the surface environment. A catastrophic threat, such as the emerging colossi of Daimalko (Near Space 74), would not only provide a reason for the sapient species of the world to retreat underground, but it also might inform how that society constructs its defenses and leverages its new subterranean home to defend against outside incursions.

Archives of Nethys

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Subterranean Worlds