Complex sentence: The identity and whereabouts of Pearl's unseen mother are one of the program's central mysteries, having never been explained directly; while supplemental materials indicate her father married a whale who gave birth to Pearl, Hillenburg is strongly against revealing this in an episode.
Split into simpler sentences: The identity and whereabouts of Pearl's unseen mother are two of the program's central mysteries, having never been explained directly. While supplemental materials indicate her father married a whale who gave birth to Pearl, Hillenburg is strongly against revealing this in an episode.