Natural Cavern Complex
Underground caves provide homes for all sorts of subterranean monsters. Created naturally and connected by labyrinthine tunnel systems, these caverns lack any sort of pattern, order, or decoration. With no intelligent force behind its construction, this type of dungeon is the least likely to have traps or even doors.
Fungi of all sorts thrive in caves, sometimes growing in huge forests of mushrooms and puffballs. Subterranean predators prowl these forests, looking for weaker creatures feeding upon the fungi. Some varieties of fungus give off a phosphorescent glow, providing a natural cavern complex with its own limited light source. In other areas, a daylight spell or similar magical effect can provide enough light for green plants to grow.
Natural cavern complexes often connect with other types of dungeons, the caves having been discovered when the manufactured dungeons were delved. a cavern complex can connect two otherwise unrelated dungeons, sometimes creating a strange mixed environment. A natural cavern complex joined with another dungeon often provides a route by which subterranean creatures find their way into a manufactured dungeon and populate it.