Throughout the game, the player will come across diaries in audio form from various players in the vision that is Rapture. Each further bend the plot's envelope, giving a deeper look into how Rapture came to be what it is, how you got there, and what each character's motives and ideals are.
While these audio diaries are not required to go through the game, they provide useful insight into the story behind Rapture and given the fact that there is no character interaction to further the plot, they are key to creating an atmosphere and plot. Also, in the PS3 version of the game, you receive the "Historian" silver trophy for collecting all 122 audio diaries in the game.
See also: Audio Diary Transcripts.
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