The Underworld is a place where all bats go when they die. Cama Zotz rules there. The Vampyrum Spectrum go to a luscious city ruled by Zotz. All other bats go to Oases. Bats that displease Zotz or are captured go to a tunnel to try to dig to the Upper World.
The Underworld is a barren desert with parched landscape dotted by Oases. The geography has nearly no true landscape, as Zotz changes the world as he wishes by sound. There are several landmarks, such as the Cactus or the Tree.