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 get out of the Pelican, you may be able to see a glimpse of the room. To get inside, you need to fly 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 tap "Y" twice really fast. You will be sent straight into the room.

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

Theories

 * It is possible that this room was originally meant to be on the level but was removed by Bungie for certain reasons.
 * 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.
 * It might be possible to get inside the room as a player through modding.