There is a set of rocks right outside the tunnel, that you can only get to by jumping, and turning mid jump. It is impossible (from what I've seen) for a zombie to reach when being fired upon. My laptop isn't letting me cap screenies, so I whipped out the iPhone. Here is a demo on getting to the spot. http://www.megaupload.com/?d=8FOQQHVL This spot is not marked as illegal, at least from what I saw.