A medium sized hole in the side of the large Cabal ship (on Mars, I think.). Me and my friend (Mars2226) found the hole and entered it. It's pretty weird, and you can fall and die through the no-collided sand terrain. We've tried to see if there is a way to get inside the ship from the outside of the map, but so far we have found no enterences except falling in between the blast doors in the "Boss room" (Y'know, the giant guy who fires tons of rockets out of his back and deals damage like crazy!). Our recent discovery has led to more people figuring out the location. I hope I didn't spoil anyone's fun posting this. (Sorry. :c) Exact outside location is mentioned at the end of the video.