At the start of "Quarantine Zone" turn directly around, and you should see a tunnel. Head down the tunnel and go straight until you exit it. Go straight across the open area, and you'll come across a broken pipe tunnel on your right. Straight in front of you is a small ledge running across the wall, above the green pit. Carefully get onto that small ledge and walk all the way to the end of the ledge to find the Sputnik skull. In this mode explosion force and melee force is greater.