It was found by Modders years ago, but someone JUST found a way to get to it legitimately, and they put it on YouTube.
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I honestly never knew that was there. That was an awesome video. Maybe someday Bungie can give us a full list of all the easter eggs they're put into all their games (or at least in the Halo games), because I know we're still missing one or two on Halo 3.