The Pillar of Autumn vacation
From Halopedia, the Halo wiki
The Pillar of Autumn vacation is a glitch that allows a player to pass the boundaries of the level and explore freely.
Through this glitch, the player can go through the glass of the airlocks that lead to the escape pods of the UNSC Pillar of Autumn. After going through the hatchway by crouching and jumping, the player 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 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.