Cowbell Skull
Skull Effect:
All explosions become much bigger.Campaign Scoring Multiplier: None
Skull Location:
The Cowbell Skull is located near the end of the sixth level, The Ark.Begin by heading all the way to the part of the level where you battle the Scarab. After defeating it, look in the middle of the desert area in which you battled it in for a fairly large structure (shown below).
Inside of this structure are several Grav Lifts. Take one of these for later. Next, head up the huge ramp, and head inside of the building (as if you were completing the level). You'll pass through a few of the rooms and corridors until you come to a room filled with lots of Plasma Batteries (shown below).
Right after this room there is a ramp that leads to the level below. In the middle of the ramp there are a series of small compartments, stacked on top of each other, that reach up to the ceiling. The Skull is located on the top most compartment.
This is where you need to use the Grav Lift you picked up earlier. Place the Grav Lift on the ramp so that it is angled and will fire you up to the top compartment diagonally. When you're in mid-air, crouch, and move into the top compartment where you can then pick up the Skull.
Or another (possibly easier) way to get to the Skull in 2 player co-op is to have both players grab a Grav Lift. Then when you reach the ramp, have one person deploy their Grav Lift on the flat area just below/in front of the compartment containing the Skull. Then have the other person deploy their Grav Lift on top of the pervious one, so that it is floating in mid-air.
Now when you go onto the Grav Lifts, they will send you to the ceiling and all you have to do is crouch and move forward into the compartment containing the Skull.