Here's the trick: Drive a Warthog up to one of these caves with the teleporter inside and park the Warthog with the back (gunner platform) facing the wall which is facing the outside boundaries. Get out and jump onto the platform (don't climb aboard, simply jump on it). Next, facing the wall nearest the back and toss a frag grenade so that it bounces off the wall and under the 'hog. You should be catapulted on top of the map and will be able to walk outside the boundaries.