Secret Room on The Covenant

On the level The Covenant, in the tower where Johnson was supposed to deactivate the tower, there is a secret room right underneath the tower controls. To get into the room, you will need to get out of the level with a Hornet and fly into the tower from the back. There really isn't much in this room. This room is believed to be what controls the elevator that the chief uses to get up to the tower control room.

Videos
To get into the tower, you will need to get our of the Covenant with a Hornet. This video explains how: Video: Halo 3 - Out of The Covenant with a Hornet

Once you have the Hornet, you will need to fly to the lower part of the back of Johnson's tower. This video explains how: Video: Halo 3 - Secret Room On The Covenant

Possible Uses Of This Room

 * This room is probably the the elevator's control and power room.
 * This room may also be the tower's power source which powers part of the shield protecting the citadel.