Minecraft wiki entry for Create_New_World, associated text: Controls the world type. Clicking the button cycles through the available world types, which are Default, Superflat, Large Biomes, AMPLIFIED, Single Biome, and, when Alt is held down, Debug Mode. When Superflat or Single Biome is selected, a "Customize" button appears, which allows settings unique to that world type to be modified. With a world type of Superflat, the Customize button options the "Superflat Customization" menu is shown; with Single Biome, the button brings you to a list of selectable biomes.