You must explore a minimum of 95% of the map in single player mode. Fly a jet at very high altitude. Start by flying over the entire offshore part of the island. Follow the edge of the underwater landmass on the minimap, which is represented by light color before it becomes very deep ocean. Next, make passes to fill in the area between the furthest offshore parts and the landmass. After "exploring" the map that is water, fly inland and pass over all the parts that are unexplored.