Secret Room On The Ark

On the level The Ark, there is a secret room right after the first cutscene. The room can only be accessed through Theater Mode. Right after the Pelican right at the beginning, when the Master Chief begins to jump out of the Pelican, you may be able to see a glimpse of the room. To get inside, you need to fly up as high as you can against the mountain on the right side of the Pelican. Once you are against it, all you have to do is quickly tap the "Y" button twice. You will then be sent straight into the room where you are free to fly around. The true purposes of this room is currently unknown.

Video
This is a video showing how to get into the secret room: Video:Halo 3 - Secret Room On The Ark

Possible Theories

 * It resembles the room in Cortana where the Easter Egg terminal is.
 * The room has a door on the opposite side. The door is a Covenant door which has blue lights on it. The blue lights indicate that the door is unlocked and can be opened if a player stands near it.
 * The room may have been part of the cutscene which took place at the end of Floodgate, acting as the stage. If one pays close attention to the cutscene, the details match to the Secret Room.