Complex sentence: In the film's backstory, human civilizations built flying cities, which were later destroyed by an unspecified catastrophe, forcing the survivors to live on the ground while the sole exception Laputa remains in the sky, concealed within a powerful thunderstorm.
Split into simpler sentences: In the film's backstory, human civilizations built flying cities, which were later destroyed by an unspecified catastrophe, forcing the survivors to live on the ground. The only remaining flying city, Laputa, remains in the sky, concealed within a powerful thunderstorm.