Land of the Dead

The Land of the Dead (also known as the Underworld) is a location where the souls of living creatures go after they die. Living people can enter the Land of the Dead by staring at a corner where two walls meet and crossing their eyes, which opens a portal. It's unknown whether this land exists in caverns underneath the Land of Ooo or if it's in some other dimension. The Land of the Dead is inhabited by many dead skeleton people who desperately want to eat flesh, as well as a variety of other strange monsters and insects. The ruler of the Land of the Dead is Death. Finn and Jake crossed over in "Death in Bloom" to revive the Princess Plant.

Features
The land is littered with bones, wrecked police cars, and flying saucers.

Escalator
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
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.

Trivia

 * Marceline, Booboo,Georgy, and Wendy all used to live in the Underworld, as stated in the episode Heat Signature.