You need to ged rid of all faded parts of the map.
If you think you have got rid of all faded parts but you did not pop the achievement you have forgot something that's for sure.
I checked all levels 3 times before I spotted the last place that I was missing.
You need to collect all 72 skulls to get to the hidden level on level 2-5 'the prismatic eye'
Pick entrance C (go to the top) when you have enter the exit you can get the last 3 skulls and get rid of all faded parts for that level also.
Thanks to NBA Kirkland he was posting this link showing us all locations!http://www.xboxachievements.com/forum/showthread.php?t=60194...
Video credits go to Bare Bum Cheeks.