Start a game in Desert, with bases 1+2.
Go up the hill by the giant base in the middle of the map, and soon you will find a ledge with two lines going up (I don't know the exact location. I think it is eagle ridge or something).
Look at the rock, then walk up along the two lines. If done correctly you can roll down the rocks and get on the other side of the map, and no one can get you unless they seawalk the rock as well.

Note: I didnt discover this either, someone showed it to me. I'm not liable for you getting banned off Live by doing this.