Out of "Halo"

From Halopedia, the Halo Wiki

In Halo 3, on the level Halo, it is possible to escape from the regular map using the Elevator Glitch. Once you get to the part with the warthogs, drive until you see a big column, with vertical orange lines going across one side. Pull over there, and jump down into the small hole there, and do the Elevator Glitch in either corner. Once at the top, there will be a mass of things to explore.