The Pillar of Autumn vacation

Through a glitch, the player can go through the glass of one of the airlocks of the UNSC Pillar of Autumn. After going through the hatchway by crouching and jumping you will fall. Upon reaching the invisible "floor" (a form of "invisible wall") you may lose some health. If it works correctly, you will be able to explore the underside of the UNSC Pillar of Autumn.

It has been discovered that the same glitch applies to the final airlock in the level. The effect, walls and area are the same but the end cutscene may stop you.

link to the video through HBO