 They roam the underwater dystopian city of Rapture, mentally conditioned to protect the Little Sisters—little girls who harvest a substance called ADAM from corpses—thanks to a series of plasmids stripping them of their humanity and free will.The player's interactions with Big Daddies and Little Sisters have been described by developer Ken Levine as the cornerstone of the game, due to the player's need for ADAM in order to gain strength in the game.Conceived of early on as a man encased in a diving suit, the Big Daddy was designed to have "that hulking metal feel of an underwater protector, so solid not even a shotgun blast could knock him off his feet."