Search isn't working, so I apologize if this is a repost. I went through 10 pages to see if this was found or not already. In any case, its on the level with teh beach landing where you climb the rope. Go to one of the fake artillary guns. There is one immediately to the right of where you come up the rope. Lay down in the center. In any direction you can crawl towards the first step up. If you keep crawling, you'll start walking through the wall. If you keep going, you'll eventually drop down below the map. You can walk around quite a bit, but can't kill anything. I only did this once so far, so but I found that getting out was a pill. Had to walk back to the cliffs so you can pop back up above ground, but got somewhat stuck. Had to wedge my way out. This glitch doesn't really do much, but if you're bored, it's something to do.