Do you know... are the tunnels mainly near cities? I was wondering because so much that I have read seems to have them connected to storm or drainage systems. Then, I thought that the really huge tunnel find was in the middle of the desert... like maybe devil's highway (or whatever the devil name is.) Tunnels are my biggest problem with the wall.
For them. As for the tunnels, I could be wrong, but I would think foot traffic would be a tip off in the middle of the desert.
In the city, you just burrow down beneath a floor and head north when you get down far enough. Nobody can see you.