This is the first step in a project I am working on and thought I would share as it is very versatile. This is a foundation I built to use under an arena or colosseum, but can be used under ANY building. Simply cover with grass or whatever you want, put a stairway where you want to enter and you have an underground complex under the building you choose. Contains several rooms, a jail, chapel, eating area, animal cages, a labrynth and more. Will keep you entertained, especially once it's covered and you can't just fly up to see where you are.
Schematic is large, a little over 160 x 160 and 7 blocks deep. I made this one easy to paste and work with. Simply stand where you want the exact center and it will paste under you. You will then be standing on a wool post marking the center. Do not knock down the wool post yet if you are going to cover the top using a world editor. The schematic will go down into the ground below you and will not work in superflat worlds unless you add several layers of dirt.
The outer circle is an 80 block radius (160 diameter). To cover the top using world edit, stand on top of the wool post in the center and type //cyl 80 1. You may want to add more lighting and build your stairway out prior to covering.
This is a project, not a completed schematic ready for use. Your input and creativity will effect the ultimate outcome. Do not leave me negative ratings for your lack of creativity.
If leaving a low rating, which I understand if you don't like it, have the guts to leave a comment to explain why so I can improve the quality of my schematics.