This is a very simple patch to the flat mapgen to create 10 extra worlds at different altitudes. Every 6km up or down is another layer of terrain, water, and decorations.
Of course there are some caveats to this.
- The surface terrain is repeated exactly as originally generated.
- It only works on the flat mapgen (but hills and lakes work).
- Biomes are placed with the actual altitude, so the default game's water biomes don't work higher than zero-world, and you get only underground biomes below. (Look below for a fix.)
- Worlds below zero-world are dark. That's built into the game. (Again, see below.)
- Caves are created based on real altitude, which could result in caves/lava intruding on your terrain below zero-world (with hilarious results).
Why would you want this? Extra space? If you limit the default biomes with y_max and y_min, you can create a new set of biomes for each world. You could have desert on one and snow on another in the same place. You could fix your mob mod to make monsters harder on each successive world. You could assign one world to each member of your family (I don't want to know who gets the dark ones).
Spoiler
https://github.com/duane-r/flat_layer
Edit: I've discovered that recalculating the lighting seems to make the below-zero layers work fine too. No problems so far. I'm including the fix in my ruins mod.
The patch