More Caves - World generation mode.

New world generation mode like "large biomes" etc that has less of the world height allocated to surface features and more of it allocated to caves. As a result the surface would have less interesting topography but there would be much more space for caves to generate. Example: Sea level 320 blocks above bedrock. The purpose of this mode would be to allow players to take advantage of more of the world height in an all underground playstyle.