Dead Worlds are places where individuals go to spend their afterlife upon death. The first mention of a dead world was in "The New Frontier" by Jake who says one must transcend into Glob World. In "The Vault," Finn is shown to have been reincarnated multiple times, suggesting that each incarnation of a person is sent to the corresponding Dead World.
9th Dead World
The 9th Dead World is mentioned in the episode "James Baxter the Horse." When an angry ghost mauls Finn and Jake, Jake thinks about their impending death. He says that, since a lot of dead people will be there, they might become separated. So as a means of identification, Jake tells him to listen for a rhythmic clapping sound he will perform.
37th Dead World
The 37th Dead World is a plane where souls go when they die. Jake's ghost went to the 37th Dead World when he was killed by the Wand of Disbursement on Mars. Death appears to inhabit this world and Abraham Lincoln was able to visit it at will. The 37th Dead World may be related to Land of the Dead, as Death is shown to dwell there in "Death in Bloom."
50th Dead World
The 50th Dead World may be a location, theoretically similar to a heaven, mentioned first in "Ghost Princess" when Ghost Princess beckons her love, Clarence, to ascend with her there. However it may also be a figure of speech as there is no way to discern where they went.
Low Level Dead WorldThe "Low Level Dead World" as stated by Jake is first seen in "Ghost Fly" when Jake is killed and appears there after having his heart stopped by BMO. It seems that the way to enter this dead world is to have unfinished business when alive which is how Jake enters as well as the Ghost Fly.
Land of the Dead
Main Article: 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 is unknown whether this land exists in caverns underneath the Land of Ooo or if it is in some other dimension.
Glob World is a place first mentioned by Jake in "The New Frontier." He explains that nothing will stop him "from transcending into Glob World" when he tries to accept his croak dream, which showed him that he would die by suffocating in space. His attempt at killing himself fails however, and the Glob World remains a mystery.