1

How would I go about putting a hole in a terrain for a tunnel or something of that nature. Just a few polygons for an opening, is all, not an entire tunnel of terrain, just a few missing polygons so i could put an external cave or something into it.

2

Take a look at http://forum.unity3d.com/viewtopic.php?t=8354

This topic is dealing with exactly this.

Hope this helps, -Larry

3

Can't do it without a really round about way of using portals and disabling the terrain collider when you reach the entrance of the "cave". Because unity uses a height map style terrain system you can't have over hangs.

http://answers.unity3d.com/questions/2132/terrain-gateways

4

Not sure if this will help, but Terrain Destruction lets you dig vertical holes at runtime!