Skulls let you unlock harder modes and are only found on normal and you have to start from
the begging. The one on the second level (Sieera 117) is when the 2 planes come p*** them
and go to the right near the water. It is on the edge.

vafdgrtbh

Go to youtube and search halo3 skulls, they're will be one that says, halo3 skulls 13/13,
go to that one and it shows you em all =)

on crow's nest (lvl 2) at the way way way beggining go look up, there will be a red beam,
it is on top of it, go up the stairs, not out the door, and jump up onto like a weapon
rack, then up to a redish beam then to the big red beam, then go to the end of that to a
gray cylander and jump on top of that, it is on top of it in the middle of it... hope i
helped

You Tube is your best chance to get skulls so go!

same as top

tysvm