Cocoon is a floating city hovering above the world of Pulse in Final Fantasy XIII.
The world of Pulse is surrounded and inhabited by lush, natural environments and dangerous monsters. These monsters attacked the humans who also lived in Pulse and so the Crystals then decided to build the city of Cocoon to protect themselves from them. It is a futuristic utopia that contains citizens who fear the world that is situated below them. Anyone influenced by the outside world of Pulse was quarantined or even exiled from the city.