One of the things that intrigues me about tales of realms below the earth, is the life (or semblance of life) that dwells there.
Many references within contemporary fantsy place vast mushroom forests within the deep caverns of the underworld. These enormous, or otherwise widespread "shrooms" are often pasty white and often pose threats to trespassers in the form of poison. Many other sorts of creatures feed off of these fungi, and are themselves very strange.
Giant reptiles similar to dinosaurs, giant crayfish, huge amoeba-like gelatin-monsters, and hidden enclaves of alien beings all haunt such underworlds. Surface races and monsters abound in these realms away from the sunlight. and so dark things can grow strong there, unhindered by light.
Humans or other humanoids that dwell in these underworlds are some of the most wretchedly fascinating monsters of mythology and contemporary fantasy. From Morlocks and similar creatures, to the monstrous ghouls of H.P. Lovecraft's dreadful imaginings.
The underworld is not a place for the weak or foolish. Adventurers might go spelunking into those lightless depths, but few come out again.