So, it's pretty simple. First, you lower terrain in a cylinder or cube until you reach the bottom of the world. Then, pick a build (one of the pre-built ones) and build it on top of that hole you just made. If all works well, that hole should now be filled with indestructible, unchangeable black 1x1s in the shape of the hole. You cannot paint it, you cannot destroy it with dynamite, jackhammer, bazooka, or the brick remover tool. What you can do is stand there and admire it, or build a castle out of these!
It's amazing
It's indestructible!
It's unpaintable!

Have fun!