Halo 2 Easter Egg - Smiley Rock

Start a game on Coagulation, and head to the red base. In front of the red base, there are three large rocks (a Shotgun spawns in the middle of them, and there are Sniper rounds there too). If you walk in the middle of the rocks and look towards the top of the rock that's closest to the cliff ahead, you should see a rather large smiley face.