Holey terrain batman!

Okay, so I've looked through the forums and resources to try and find an answer to this. I *thought* had seen a forum post regarding it previously, but I didnt bookmark it and cant find it now.

Anyway, I need to create interiors that go under the terrain. For instance, an underground tunnel that goes between buildings. I dont need to actually dig the terrain so that it has walls or anything, I just need to delete parts of the terrain so that it is not visible or collidable.

I can create the interior so that it sticks through, and then when I enter an interior the terrain is not rendered inside of it, but the collision detection still applies. (And I can still see the terrain inside of it if I am looking from the outside)

Maybe there is some way to flag the collision detection not to include the terrain in its tests when the subject is inside of an interior?

I'll take a look at the source code and try to find a way to deal with it, just wondering if there is already some way to do this that is in the engine and I havent been able to find it.

Any ideas would be great.


Thanks
Gee