Unknown Skull In Halo 3

In Halo 3, there is a unknown skull in the level Cortana. It can be seen on top of the roof in the Fusion Core room. There is a ledge where you can jump onto the level above the main door (It is recommended that you kill all Flood forms in the area and attain the Deployable Cover for higher height). Look around the upper area until you find a skull with pile of bones around it. This skull can also be seen by utilizing the Theater Mode. Once your character reached the Fusion Core room, detach the camera and search across the roof. Above the main door, you should be able to see the skull.