Complex sentence: The subsequent investigation into the disaster revealed that control mistakenly believed that Flight 1308 was out of holding pattern, believing the aircraft was already located over sea, while in reality it was located some inland, over mountainous terrain of Corsica.
Split into simpler sentences: The subsequent investigation into the disaster revealed that the control mistakenly believed that Flight 1308 was out of holding pattern. The control believed the aircraft was already located over sea, while in reality it was located some inland, over mountainous terrain of Corsica.