A dungeon lies between the overworld and the underworld. The underworld can be run in the same way the overworld is, but you will get best results by paying attention to the difficulties of life underground. Calories and light are much more precious, but treasure more plentiful.
Fungi are the staple of fictional underground ecosystems because they need no sunlight. The fungus jungles are the densest and most diverse pockets of biomass you will find in the depths.
Lichen on the other hand should require light to live. In truth these are not lichen but a similar symbiote of fungi and algae.