Complex sentence: He takes her to his Fortress of Solitude in the Arctic, and shows her the traces of his past stored in the energy crystals of the fortress, one of which is the green crystal that created the Fortress and opened Superman's contact with his parents.
Split into simpler sentences: He takes her to his Fortress of Solitude in the Arctic, and shows her the traces of his past stored in energy crystals. One is the green crystal that created the Fortress and opened Superman's contact with his parents.