The game starts with the player underwater following a plane crash in the ocean, surrounded by debris. No introductory cut scene is displayed explaining how this happened, leaving the player to decide for themselves why they were on the plane. Nearby, a lighthouse is sticking out of the water, in the middle of the ocean. Upon getting inside and traversing the internal stairwell, the player finds a bathysphere, with a corpse inside. Upon removing the corpse and descending in the bathysphere (having nowhere else to go), the player eventually reaches an underwater city on the ocean floor.
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http://en.wikipedia.org/wiki/Bioshock