High Ground vacation
From Halopedia, the Halo wiki
The High Ground vacation is a "vacation" glitch that allows you to travel out of the level's boundaries.
You need to be on Forge to do this, and have at least two players.
To get out of High Ground, Player 2 must get into the backseat of a Mongoose. Player 1 then turns into Forge mode and picks up the Mongoose (You Have to be in the back seat to do this, or you will climb out). Player 1 then pushes it behind the rocks on the left (if they are facing towards the sea) and Player 2 climbs out. Then Player 2 turns into forge mode and can climb up to the edges of the map.
Note: It is easier to do this glitch if you overload the map. This glitch also works with the Warthog
Spawn a teleporter outside the map, and spawn a second one on the map. This way can be easier if you don't understand the first method.