The Land of the Dead
The land is littered with bones, wrecked police cars, and flying saucers.
A large, downward-moving escalator leads to the entrance, which is guarded by the Gate Guardian.
The River of Forgetfulness
This river flows throughout the land and whoever drinks from the river instantly loses all of their memory. Jake was shown having this happen to him. Also, the river goes into Death's castle. The River of Forgetfulness is a reference to the Lethe, a river in Greek mythology that erased a dead soul's memories when drunk so the soul could be reincarnated.
Death's Castle is just beyond the River of Forgetfulness, and is comprised of light. Inside is what resembles a Zen garden with various boulders about the landscape, and there is also a stage with several instruments where Death holds musical contests with those who seek to get something from him.
- Marceline, Booboo, Georgy, and Wendy all used to live in the Underworld, as stated in the episode "Heat Signature".