Well if they were going to do it Reasonably, they'd Put in two fences, One Fence on the Border, Heavily Land Mine behind the First Fence, and then say 5km north of the first fence build another fence. If they make it through, Congrats! If Not... well... But, I don't support the idea of the fence anyways, I say any Americans with common sense would just annex Mexico and be done with the trivial Immigration fiasco altogether.