How do i make a model go into a moutain

#1 Check out the documentation on Constructor. There's a way to make individual triangles invisible in editor.

#2 There is an option to skip rendering part of terrain in *.mis file if DIF interrior is crossing it.
The other way as far as i know is, you can delete terrain faces from within Mission editor, making them able to pass trough.

#3 There is an option that is something like:

allowSubTerrain = 1;

Or something like that, basically it make the terrain invisible wherever is touches, and allows objects to pass through. that area.

I will post back in a minute about the code you need in your datablock.