You can fall through a mountain in Khopshef Province. The coords are approx 1193,1131.
You will have to walk along the wall (heading towards the west) and there is a spot where you will kinda get stuck or run over like a bump. You should be pressing "w" and "e" at the same time so that your trying to walk into the wall. Keep trying at different heights and you will eventually fall though...
I noticed jumping helped a little.