Complex sentence: The investigation into the loss of "Taihō" had revealed that her avgas tanks had sprung leaks after she was torpedoed and fumes from the leaks had penetrated the cofferdam that was supposed to isolate the tanks from the rest of the ship.
Split into simpler sentences: The investigation into the loss of "Taihō" had revealed that her avgas tanks had sprung leaks after she was torpedoed. The resulting fumes then penetrated the cofferdam that was supposed to isolate the tanks from the rest of the ship.