i tested the walls with a rocket launcher...there are places that you can fly through. I've tried hard to get the DODO that high, but I can't do it. Also tried walking, flying tank trick, stacking cars on top of each other and climbing, driving the DODO up to the dam and taking off from there, driving a Cartel Cruiser straight up the hill at top speed, pushing a boat with a car into the small lake behind the dam (don't know what i was thinking there), walking all the way from the far side of the hill to the north of portland, flying behind the enter island (near the secret island) and trying to go THROUGH the mountain, and following the DODO with wings that hangs out around there to see if it can get to the top. All have failed so far.