Backwash

Summary
Backwash is a multiplayer level in Halo 2, similiar to the campaign level on Halo: Combat Evolved called 343 Guilty Spark. It is a large swamp set on Delta Halo. You can tell it is Delta Halo, not Alpha Halo, as 2401 Penitent Tangent can be seen floating around and humming to himself. It is actually stated by the developers of Halo 2 that Backwash was originally set on Alpha Halo and based directly on the Halo: Combat Evolved campaign level,"343 Guilty Spark". It is also stated that they originally had 343 Guilty Spark floating around but decided to change him to 2401 Penitent Tangent because 343 Guilty Spark's blue glow could too easily be mistaken for a Plasma Grenade flying through the air.

Removal
Backwash is the only map to this date to be removed from matchmaking on Xbox live, due to problems with the Xbox 360 loading the fog.

Hidden Message
There is a way to get out of the level, but it requires super-jumping, a modder or a spectre. With the modder, get the him/her to superjump (a certain hack) over the invisible wall on the hill, then get them to stand on top of the base (red or blue). Get them as close as possible to the wall and then sword cancel up onto a small spot on top of the base. You can now sword cancel (try it a lot) through the wall. Then, walk through something that looks like a large amount of roots (surrounds the outside) and look for edge of the level. Then walk around until you find red ground. The message is very large, written in blood, and says: Lookin a little rough out here. (Possibly because of the mold, fog, etc.)

To do it with the spectre, go to blue base and jump on a small ledge that is right next to the invisible wall. Have the driver put on a severe handicap (if you don't have an overshield) then ram into you. If done correctly you will be smashed through the invisible wall. Then, as with the modder, have anyone who wants to get through sword cancel outside.

Images
