Hi Fishman
I saw a thread somewhere about a tunnel network addon for use in an Afghanistan scenario, not sure where though. Search under the Addons forum or the library, or try Googling for "tunnels OFP".
Essentially, the tunnel network is just a standard addon building, but with only one or two ways in and out and of course no windows !
You simply place the tunnel "building" somewhere on your map well out of game play (say, an offshore island) then create a trigger point that automatically zaps the player into the building.
I think the addon also had a "tunnel entrance" building to put the trigger on - you essentially walk a few feet into a darkened hole and are then zapped into the darkened tunnel building on the other side of the map. You don't see this of course - as far as you're concerned you are still crawling through the same dark tunnel.
Ditto in reverse to zap back to the surface.
Pretty clever really !
I'm not sure whether or not the tunnel "building" is modular or not (I don't think it is), but it looks like a pretty cool addon nonetheless.
Cheers
Roni