By Dan Zuccarelli . October 8, 2007 . 12:20pm
So there's not much to unlock in Halo 3, but if you search through the game (or go online and watch the video) there's 13 hidden skulls to find. Each one unlocks an achievement and if you get all of them, you unlock this Ninja Gaiden inspired "Hayabusa" armor to use in online multiplayer.
These photos are from users on thebbps' forum.