Secret Room On The Ark

200px|right On the Halo 3 Campaign level The Ark, there is a secret room right after the first cutscene.

Description
The room can only be accessed while watching a saved Film in Theater. Right after the Pelican at the beginning lands, and 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 immediately pause the Film, and 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 Y twice. You will then be sent straight into the room where you are free to fly around. The room is actually the bridge of the Assault Carrier Shadow of Intent, commanded by Rtas 'Vadum. The room is seen in three cutscenes: The one between Floodgate and The Ark, the one at the beginning of The Ark, and the one at the end of the game, where the Arbiter and Rtas 'Vadum are discussing the future.

Purpose
This room is not actually an Easter Egg; the room itself isn't a glitch either, though the method of accessing it is a glitch. Despite many wild theories on the subject, the truth is, the room serves an actual purpose.

In the Halo series, cutscenes are not pre-rendered, they are rendered using in-game resources. This means that the cutscenes are not pre-made and added in, but they, in fact, take place in the level itself. Therefore, everything you see in a cutscene has to be somewhere in the level -- not necessarily where you can see it or even access it, but somewhere. This is why if you throw a grenade right before a cutscene, the burn mark will be visible (depending on what you used and where you threw it) and why dead bodies in cutscenes often vary. Cutscene rooms are often placed below the level and large scenes like space battles sometimes happen in a completely different and separate part of the level. The placement of the bridge and the glitch used to get into it are pure coincidence, and if the bridge was placed in a different place the glitch would not have the same effect.

Trivia

 * The room also appears in Floodgate, though there currently is no known method of accessing it in Floodgate from the Theater without using Pan Cam and Pan Cam Coordinates.
 * It should be noted that the room is basically an uninfected version of the Tilt Skull room on Cortana. One highly likely theory is that this room was built first and for Cortana, the environment designers modified it slightly and added Flood biomass.