Rapture Historian Achievement
Find 100 audio diaries.
You will find audio diaries through each section of Rapture. They provide a lot of back story and depth to characters that you might run into while exploring. Make sure to check every room you enter for these diaries.
There are 128 audio diaries throughout Rapture so missing a few isn't an issue, but don't just ignore them. You won't be able to come back and collect them once you move onto the next level.
Audio Diary/Power to the People Upgrade Station Guide