Xbox Halo 3 Hints: Skull locations
There are 9 skulls to find for the Meta Campaign, one for each level. You have to start each level with difficulty set higher than Easy from the beginning. The Fog skull is on "Floodgate" not "The Storm".
And when you've got them all, you will definitely be entitled to award yourself a
Halo 3 Master Chief Belt Buckle from Hot Topic. Very nice.
