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GameMastery Guide / Adventures / Wilderness / Environment

Underground

Source GameMastery Guide pg. 223
Caves form under a huge variety of circumstances: by the action of waves along the shore, internal erosion from underground streams, or acidic air and water, as well as by tectonic action. Unless frequently used as a passageway by cave dwellers, a cave has rough and erratic floors, and its passages expand and contract along their lengths. Drop-offs are frequent—travelers without safety lines are almost guaranteed to suffer falls. Even worse are cave-ins, which come with little warning and are occasionally the result of traps set by cave-dwelling creatures. Movement underground is difficult without a light source, and subterranean creatures will either be attracted to the light or squirm away from it as quickly as possible. Food is difficult to find in caves unless a creature has evolved specifically to eat subterranean lichens and molds—a prime reason for most creatures to live aboveground. Creatures that have adapted to the perpetual darkness learn to navigate by touch, smell, taste, and sound, and they are extraordinarily aware of intruders in their realm.

Dungeons are described in greater detail here in the Pathfinder RPG Core Rulebook and here in this book. Hazards in the Core Rulebook that can help liven up underground encounters include cave-ins, suffocation, falling rocks, darkness, and dangerous molds and fungi.