Dark Armada — Colin D. Peel

When Steve Redman, an American engineer working in Australia, goes to the Solomon Islands to investigate the death of a pilot in one of his employer's planes, he expects to ask a few routine questions, write a cut-and-dried report, and fly back home. But Steve doesn't know what the reader already does - that the plane blew to pieces when it flew into a mysterious mist high above an uninhabited part of the Pacific and triggered a giant explosion that to an onlooker would have rivaled an atomic blast.
Steve and the pilot's young widow, led by a vague account from a pair of native fishermen, begin a search for the cause of the explosion - a search that leads to the revelation of a plot threatening countries half a world away and to violent death for people close to Steve himself. And he is next...