Complex sentence: The island is supposedly destroyed when Alfred sets off the self destruct system for the island, however the device obviously fails to detonate fully; the training facility is left partially standing, although badly damaged, as is the airport and the Harrier facility.
Split into simpler sentences: The island is supposedly destroyed when Alfred sets off the self-destruct system for the island, but the device fails to detonate fully. The training facility is left partially standing, although badly damaged, as is the airport and the Harrier facility.