Dungeons, burrows, caves, and realms all share some common hazards that are created by their location under the surface of the earth. The following dangers are relatively rare, but each of them can be life-threatening to a character deep below the surface of the world. Cave-ins are a constant problem to creatures living underneath tons of earth and rock.
Cave-ins can be triggered by erosion, or by sudden shocks such as earthquakes or explosions. Of course, characters or creatures directly underneath a section of dungeon or cave that caves in suffer damage from the material falling on them, and are often pinned by the weight of this material, or trapped in a small area with little air. Dungeons are often protected against cave-ins by shoring up the passages with timbers or stone arches. Caverns do not have the benefit of such engineered protection, and realms are generally too vast for such protection to be practical. Because of their vast size and durability, the realms of the underearth are rarely subject to cave-ins. If a cave-in occurs in a realm, however, its potential for disaster rivals that of the worst hurricane or tidal wave ever to ravage a surface land.
Floods are yet another natural phenomenon with potentially disastrous results. Because of the cramped nature of underground environments, flood water has little chance to spread out and disperse its effects. Indeed, underground corridors may well fill with water, making drowning a virtual certainty for any airbreathing creature caught there.
Fires threaten the well-being of underground dwellings in a double sense, for they consume combustible objects in their path, and also use up the precious oxygen that definitely exists in a finite supply. Fortunately, at least for fire prevention, wood is not a very common building material underground. The difficulty of transporting wood into a dungeon and its tendency to rot in damp conditions both serve to discourage its use. Certain tasks, however, such as the shoring up of a corridor, are much more easily accomplished with wood than with stone.
Poisonous Gases present an unseen but real menace to all creatures that must rely on oxygen for life. These gases can be of natural origin, such as the sulfur dioxide commonly created around areas of volcanic activity, or can be of magical or manufactured origin. Obviously, the use of poison gas as a weapon of war is greatly enhanced in the constricted conditions of the underearth.
Volcanoes are a major cause of subterranean fires and poisonous gas emissions, as well as other problems. Often occurring in conjunction with earthquakes, a volcanic burst can fill inhabited corridors with lava or steam, or close off escape passages or popular transportation routes.
