Originally Posted by MCerberus
Post Apocalypse Earth = Adventure Time is one of those 'And now I cannot unsee it'. There's ruins of modern cities in a lot of places. But everything's so happy about it.
Personal theory is that Ooo shares a border with Equestria.
I think that Pen also said no one realizes it. But even if they did, what's there to be sad about? There's evil but there's heroes too, and we can tell that Ooo is pretty awesome even with all the monsters.