The Land of Ooo, in which 'Adventure Time' is set, is actually a post-apocalyptic Earth!

The Land of Ooo, in which the animated television series ‘Adventure Time’ is set, is actually a post-apocalyptic Earth. It is the home continent of Jake the dog and Finn the human and they share it with their strange friends and enemies. Adventure Time became a viral hit on the Internet and that is why Cartoon Network picked it up for a full-length series. It previewed on March 11, 2010.

According to Pendleton Ward, the creator of the show, the land of Ooo came into being after the global disaster of the great Mushroom War that left Earth destroyed. This is why there are so many ruined pieces of modern technology scattered across the land. Ward said Finn may very well be the only human left in Ooo. The other humans were either all killed during the Mushroom War, or they mutated into the varied inhabitants of the planet as a result.

There are many languages spoken in Ooo besides English. The Rainicorns speak Korean, the Flame King speaks some French and Spanish, Latin and German is spoken by some of the characters in certain episodes.

The land is divided into many different kingdoms. The most important of these kingdoms are the Ice Kingdom, Candy Kingdom, Castle Lemongrab, Wildberry Kingdom, Lumpy Space, Fire Kingdom, and Cloud Kingdom.

(Source)