Location: At the start of the level, turn around and head through the tunnel up ahead. Walk along the edge of the cliff and then move onto the narrow walkway that is attached to the mountainside. The skull is floating at the end of the walkway.
With this skull active, the weight of most moveable objects decreases, so explosions can send them flying further. This effect remains active until you turn off your Xbox and reload the game. To re-activate this effect afterwards, you’ll have to collect the skull again.