Imaginationland is another dimension. It can only be accessed with the "Imagination Song".
The entire population consists of fictional characters from fairy tales, video games, cartoons and mythology. The antagonists are separated from the rest of the Imaginationland with a giant wall. Terrorists destroyed the barrier between the good and evil characters in order to let people's imaginations run wild. Imaginationland was destroyed by a nuclear missile with everyone in it, including visitors, with Butters Stotch as the sole survivor. As the chosen one, he used his imagination to revive everyone and put the bad guys behind the barrier.