The Underground Passage is a secret entrance into Bargate Prison in Fable, Fable: The Lost Chapters, and Fable Anniversary. This is used in the quest to break your mother out of prison, the passage itself is inhabited by a hefty amount of undead as well as many of the prison guards. If detected by the guards they will attack. During this time, most of them will ask you how you got down into the prison, considering how the passage has been blocked off for such a long time.