They have always existed, but have been cloaked from the visual light spectrum and from radar by beings that have lived on Earth in secrecy for millions of years.
Mankind decides to investigate and discovers that the visitors are anything but friendly.
They are reptilian, distant cousins of the dinosaurs, and are using the islands as a series of experimental laboratories and training areas. Their being there is unknown. The UN sends in several Marine-Units to check out what is happening on those islands. Once found, the officer will tell the player where the objects that are needed to complete the task are. The Gamer has to fight his way through the combat field to find the required objects.