To unlock this achievement you need to find all of the secrets in the game. The secrets in this game are secret area's and you need to enter them to 'Discover' them. Please refer to the playlist of videos below for a walkthrough of each level including the secret areas.
Please note that these videos are for the PC version as no videos for the Xbox 360 version exist. Some of the levels are extended on the Xbox 360 version, and there are 3 more levels too that have no secrets and are missing from the videos below, but all of these videos show the same secret locations that you need to unlock the achievements.