One of the replayability features in Halo 3 are “skulls”, special doodads hidden in obscure places in the various levels for you to find. Finding them is fun in itself and once you’ve found them you can use them to unlock modes that make the game harder. I can’t imagine finding these on my own, so I went online to Halo 3 Planet, Team Xbox, and Hushed Casket to find them. Here’s a list of their basic locations, spoilers ahead.

Blind Skull*, Sierra 117 (L1)
Iron Skull, Sierra 117 (L1)
Grunt Birthday Party Skull*, Crow’s Nest (L2)
Black Eye Skull, Crow’s Nest (L2)
Tough Luck Skull, Tsavo Highway (L3)
Catch Skull, The Storm (L4)
Fog Skull, Floodgate (L5)
Famine Skull, The Ark (L6)
Cowbell Skull*, The Ark (L6)
Thunderstorm Skull, The Covenant (L7)
IWHBYD Skull*, The Convenant (L7)
Tilt Skull, Cortana (L8)
Mythic Skull, Halo (L9)