[maximus_asinus]

I would also suggest that the player should not be able to see what is several layers below them. Maybe just the dirt/cavern areas without minerals until the player is near.

Also how are you going to setup an overworld? Is there going to be a few hundred horizontal levels with the possibility of digging down (or building up)? Would there be multiple overworlds? Would players be kept separate from one and another or on one overworld?