Face on Gravemind
To see it, play the level on any difficulty to the second part where you are outside and go to the place where the Special Operations Grunt is carrying the Fuel Rod Cannon. It's not where the Anger Skull is, but a place like it. Go out the door and kill the Brute, the Jackal and the Elite, then go to where the Jackal Sniper was before you killed him, then turn around. On the wall beside the door you came out of, should be a crocodile with something in its mouth. It is somewhat unclear, although using the scope on the Sniper Rifle will allow you to see it clearer. The face is made out of the lines on the wall.