For this achievement, you must locate and open all storage lockers in the main game of Doom 3. Good thing about these is most of them you will encounter throughout the game without having to do much serious searching, which is pretty much just like the other collectable achievements.
NOTICE: This achievement appears to be glitched in a good way, unlocking earlier than expected. Some have reported it unlocking in Delta Labs 3/4 instead of later on.
It would be best to have up THIS thread with the storage locker codes while you are going through the game so that you do not have to search through your PDA to find the codes.
Please visit THIS post for a spreadsheet of all collectables found in chronological order.