diorama base

As was mentioned, this is a good way to build up countours over which you can apply Celluclay or other material, but it would not be adequate for a base. It's too weak and dissolves when it contacts any glue except white glue.

Ya, hes definately right, styrofoam is the way to go.  You can make hills, trenches, rivers, ditches, all kinds of stuff since you can form it easily with a knife, unlike wood or something

Styrofoam makes an excellent substructure material for dioramas, but you need to support it with something more rigid underneath, like a picture frame, wooden plaque or a piece of MDF. You can carve styrofoam into the rough shape, then cover it with celluclay or plaster.