I support Sirbomber, hey look at the facts the british dug a tunnel and demolsihed an entire mountain at the same time to make the chunnel so why can't you dig a gap through a darn cliff? It may take a painfully long time but when you are digging into an area that is rich with minerals you tend not to care! And with the tunnels, use OP1s ideas and make it so they have to biuld an elevator shaft... my idea for the elevator could be used here, once the shaft is complete the robo digger biulds the tunnel but the exit shaft has to be dug from the surface also, so if they biuld under your base they can't exactly pop up unless you're just going to sit there and watch them biuld a elevator on your base perimeter... come to think of this if we used an elevator to lower units into the tunnel it would require a tube connection and power so if they do somehow manage to biuld an exit shaft in your base then just blast the elevator which would probably collapse the entire shaft. Of course you could always just biuld a bunch of guard posts around it and blast the vehicles as they're coming out (just to rack up some kills).
The disadvantage could be for either the defender or the attacker if only a single unit at a time can come out of the tunnel. MMMMWWWWWAAAAA!
