Complex sentence: The mystery of the second Terra's origins was addressed in the 2008 "Terra" miniseries: she had been born in an underground world called Strata and sent away by its members to establish relations with the surface world.
Split into simpler sentences: The mystery of the second Terra's origins was addressed in the 2008 "Terra" miniseries. She had been born in an underground world called Strata and sent away by its members to establish relations with the surface world.