Halo 3 Cowbell Skull

Effect: Explosions become much larger in the game.

Location: The Cowbell skull can be found on The Ark level.  It is quite far into the level, so it takes a bit of time to find.
 Start by heading all the way to the part of the level where you battle the Scarab.  After defeating it, look in the center of the desert area which you battled it in for a fairly big structure (it is the building you see if you are at the bottom of the ramp you are supposed to go up, and turn to look in the opposite direction).  Inside this structure are several Gravity Lifts.  Take one of these for later on in the game.

Then head up the huge ramp and head inside the building (the one with the cartographer inside).  Go through a few of the rooms in this building and you will eventually come to a room with a lot of explosive crates in it.  Right after this room is a staircase/hallway that leads to the level below.  The skull can be found at the top of this room, at the point where you turn on the staircase to head for the next part of the slope.  There are four platforms above you, the skull is on the highest of these platforms.  Use the Gravity Lift to jump up to it and there it is.