"The true story behind adventure time is actually really damn depressing. It takes place something like 1000 years after a nuclear war, which wiped out all humanity besides the ice king, who was horribly mutated. Everything you see is an abomination brought upon by nuclear waste harming and mutating the environments and animals. They actually show the damn war and following nuclear winter in a few episodes. F*ckin depressing, man" -
The Nuclear war he mentions is the "
". If you are wondering about the Opening Sequence, the first thing you will see is a landfill filled with military refuse, and beyond it is the land of Ooo.
Here are more.
"Adventure Time passes in a post-apocalyptic world where humanity destroys itself with a colossal nuclear bomb (you can see nukes in the opening) , after this bomb (and maybe not just one) falls, human mostly died and who survived went to the underground, to protect themselves from diceases created by the effects of the bombs. But, when they were underground, they did not know that above their heads the world was changing, and the bomb, even reaching the sun, opened diferent dimensions (Nightosphere, Lumpy Space, etc.) and make the habitants of these diferent dimensions to start living on Earth, the creatures and humans that survived the war and still were in the surface were transformed into mutated creatures because of the radiation (Candy people, Undead, Raincorns, Talking Dogs, etc.).
In "Ocean Of Fear", when Jake and Finn go to under the sea, in the background you can see a complete ruined city underground and a place where was supposed to be a traffic lane with buried cars on it. When they try to reach Lich before he goes to the "Power Well", you can see in the surface a unused nuke before they enter in the subway.
There are many evidences of this war in the cartoon, you just need to find them". -
Well that's copied and pasted from an article. If you follow Adventure time, than I think you would have already starrted getting that vibe. Remember, Ice king used tobe a regular guy until the Apocalypse when he had to use the crown to protect himself and kid Marceline. But it drove him crazy.