Hello my fellow minecrafters of the world! Milkman here again to show you the SuperFlat++ mod for Minecraft 1.3.2. Dont you hate it when you start your flat world and it looks cool and all but there is a problem which many people hate! The fact that is only 3 blocks deep. Well I have the solution for you! The Superflat++ mod allows you to pick the dimensions of your Superflat world, neat eh? This mod is really simple and I thank the creator for making this because it was so annoying with my Superflat world being so shallow 
So to basically sum up this adds a new textbox that allows you to enter a custom world Height for SuperFlat your able to enter any number from 1 to 128. Here is a picture show how deep you can make it and how to set it!
SuperFlat++ Mod for Minecraft 1.3.2 Main Features
- Custom Superflat world
- Easy GUI
- Very convenient!
And also here is a wonderful video by GameChaps showing off this mod 
SuperFlat++ Mod Version 3.6 for Minecraft 1.3.2 Changelog
- Forge update 3.6
- More buttons under world options
How to Install SuperFlat++ Mod for Minecraft 1.3.2
- Download and install Modloader
- Download and install the SuperFlat++ (extract the files and drag them to you minecraft.jar)
- Configure your options and profit!
