Superflat

The flatland was first introduced to Minecraft in Snapshot 12w01a. The flatland is a world type feature named "Superflat".

The terrain consists of one layer of bedrock and three layers of dirt; the surface of the world is completely flat and at height y = 3.

Mobs spawn as normal, including slimes; however, there is no “void fog” as would ordinarily be caused by being close to the bottom.