Complex sentence: As an unspecified global catastrophe looms, an underground city, Ember, is constructed to shelter a large group of survivors, while a small metal box intended for a future generation of Emberites is timed to open after 200 years.
Split into simpler sentences: As an unspecified global catastrophe looms, an underground city, Ember, is constructed to shelter a large group of survivors. A small metal box intended for a future generation of Emberites is timed to open after 200 years.