The caldera, or crater, almost enclosed by the island of Santorini ranks as one of the natural wonders of the world. The volcanic explosion that created it some 3,500 years ago changed for ever the topography of the Mediterranean coast. The flooded basin is four miles in diameter and has been linked with the myth of Atlantis. The sheer cliffs are more than a thousand feet high and plunge beneath the sea for another thousand feet, creating a perfect natural harbour.