Edit: Here is a video walk-through of the bunker!
https://www.youtube.com/watch?v=QFynhHBPzm4&feature=youtu.be
http://i.imgur.com/i7KJeJG.jpg
http://i.imgur.com/jOtxWlb.jpg
Here is a tutorial on how to do it.
http://www.reddit.com/r/DestinyTheGame/comments/2b80fh/hidden_area_glitch_seraphim_vault/
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I know the knights are referenced many times and they are the ones in the basement of RAS-2 on the surface, the ruined building housing the second amplifier in the story mission where we learn of Rasputin's existence. The area people keep finding their way into is the Seraphim Vault if I am not mistaken. This area is content that we were informed would be coming soon. In reality, it was content we probably should already have had at game release, but I won't beat that old dead horse anymore.