On the strike mission (I did it on level 8, but I don't think it matters), if you go backwards all the way to The Grottos then drive your sparrow into the water at the far end, it will let you onto the water without dying. You can then head towards the Forgotten Shore and jump over an invisible barrier (this part is a bit tricky, try to do it about 6-10 meters from the shoreline). Once you get there the game still thinks you're in The Grottos. This causes lots of scenery to not appear and allows you to fall under the map just past the edge of the water (about 6-8 meters inland). From there stay on the water and go back towards The Grottos and it will load that areas scenery but you'll still be below it. Then head back toward The Forgotten Shore and it will load properly but again you're below it. Now there is a dead ghost visible below one of the ships (the one in the room with the Hallowed Knights). Above it is a square concrete platform that you can jump onto then it should load the Bunker RAS-2. If you carefully make your way around and above the structure you'll find a place called Seraphim Vault. Once I reached it, (above it anyway) I had my brother join my Fireteam and he spawned inside. There is another dead ghost in the far tunnel and some really cool scenery. I think this area links up to another (just past the dead ghost) but I can't seem to get into it yet. Edit:Now with video's (AND A TUTORIAL) (NEW)Tutorial on how to get there: https://www.youtube.com/watch?v=RZmxNjRcI9Y (OLD)This shows where and how to fall through the map: https://www.youtube.com/watch?v=MXeaWHnD85A (OLD)This is above the vault: https://www.youtube.com/watch?v=-IeTKDwbssI (OLD)And this is the inside: https://www.youtube.com/watch?v=q60BTY0E25o