In "Sacred Icon" there is a semi outdoor area which has the same type of area right next to it. There is a large sentinal in front of each one. Go past this area and down the piston and you'll be in a vary dark and dank flood infested area. There's a stack of 6 or 7 boxes as you enter the room. Use them to jump up to the landing above. There are a handful of alcoves on your right hand side. The skull is in the third alcove, floating above a dead human flood. When you are in this mode flood are invincible when driving vehicles.