Minecraft wiki entry for Overworld, associated text: The Overworld is composed of a large number of terrain patterns, called generated structures, whose arrangement varies widely from one seed to another. The exact structures are unique to each world, while the types of structures that can be generated at a given place are determined by the biome type. Structures are meant to represent real-world equivalents such as mountains, caves, and lakes. "Impossible" (in the real world) formations, such as floating islands, can also be found throughout the Overworld.