Complex sentence: The incident was immediately covered up at the time and explained away as 500 pound military practice bombs being detonated, although it wasn't until the 1980s when the Air Force confirmed that it was a nuclear incident.
Split into simpler sentences: The incident was immediately covered up at the time, and explained away as military practice bombs being detonated. It was not until the 1980s that the Air Force confirmed it was a nuclear incident.