Creating a cave?

[Xrak]

I have been wanting to create some caves in the flavor of the Minoc caves.  That is, a mountain, with a mouth opening that someone walks through and is then in a cave for mining, killing, whatever.  I loaded up CentrED today, and tried working it out... So far, have not been very successful.  Every time I try to place a cave floor at say a 0 Z value, the editor things I want to collapse the mountain to 0 level.  So, it creates this monster valley.  I can't seem to get it to put a second layer in at z=0.  Any ideas how to do this?  Theoretically, you should be able to create many layers on top of each other.

[Hitman]

You have to use statics for the ground of the cave. There can always only be one map tile per x, y coordinate.