This feature was exclusively part of a joke version and is not currently planned for addition to regular Minecraft. It is documented here on the wiki due to being officially-released content by Mojang Studios, even if not present in a final version.
The biome consists of a somewhat mountainous terrain and rivers. Different shapes made of one or more different types of blocks generate across the biome. No mobs spawn.
One of the rivers that can generate in the biome.
Shapes generating in the sky.
A cube generating underground.