FOR those ' in the know,' Cocos Island calls up the most romantic possibilities of almost any island in the world. Cocos, the isle of buried treasure, is a tiny island some 400 miles off the coast of Costa Rica, in the Pacific. Somewhere under its cliffs and sands, among the jungle of grasses there, lies treasure to the value of millions of pounds, gold and silver, weapons and images, jewels and plate. One of the treasure troves dates back to 1685, when Captain Edward Davis , partner of Dampier and one of the most famous buccaneers of his time, had sacked the city of Leon, in Nicaragua. Another dates from the early nineteenth century, and is the treasure of the pirate Benito Bonito. Whilst a third, and the greatest of them all, is the famous treasure of the city of Lima, which was buried on Cocos in about 1821 by a certain Captain Thompson, a Scots morchant shipper who turned pirate and joined Bonito. Captain Malcolm Campbell, the famous racing motorist, will describe how ho went in search of this stupendous treasure.