The world from Adventure Time. It’s heavily implied to be a post-apocalyptic earth, which is wildly mutated from the present day by an atomic war. IIRC, the stated backstory is that the land of Ooo is what remains after the Great Mushroom War. We see glimpses of the world immediately following the war, and it looks like a Fallout situation, with packs of survivors, radiation exposure, etc… And eventually, it settled into the Land of Ooo.