The subterranean civilization is living in a massive cavern that is lit up by bioluminescent fungi. The cavern is so big that it has its own weather system, with clouds and rain. The subterranean people have built their homes into the sides of the cavern, and they use the fungi to light their homes and streets. They have also cultivated gardens of edible fungi. The subterranean people are a peaceful society, and they welcome visitors from the surface.