Halo 3 - Iron Achievement

Find and claim this skull while playing on Normal difficulty or Harder

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After Freeing Johnson and killing all the covenant that come in with the phantoms, walk to the Pelican that comes to pick you up. Face the cargo bay but instead of hopping in, turn left and follow the path. Once you hit the wall jump up to your right, eventually reaching a catwalk sort of thing. Walk all the way to the end, take a left, then a right, then at the end of this little bit to the right in a dark corner. Turn your light on and you should see the skull. If you cant see it you're probably standing on it.

Pick up the skull as you would with a regular weapon, and the achievement will unlock.

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All skull location video by RoosterTeeth