Idk if this has been posted before, and yes it is kind of old but anyways...
First you need to go to the bravo base and go to this wall.Look at the radar for exact location :]
Run at the wall, nothing else all you do is run and you will go in.
After you get in jump to that black room.
The walls in the black room are just for show, you can jump through them, so after doing so you can get outside the map, here are a few places you can go and see.