A world whose lands are made up of huge terrain spheres that rotate constantly with most portion underwater. As time passes, the shape of the bodies of water change, landmarks shift inside the new border lines, and mountains tilt to different degrees. Land dwellers are gypsies that can never build anything permanent, and somewhat ironically, the only stable settlements are large structures built at sea.
The World is actually a number of continents surrounded by oceans, which are in turn surrounded by mountains. The oceans rush off the edge of the plateau on which the World sits into the Outside...