Santorini island, Cyclades, Greek: Oia village and port seen from the sea. Oia village is on the slope of the collapsed caldera. Santorini is an island located in the southern Aegean Sea, about 200 km southeast from Greece's mainland. It is the largest island of a small, circular archipelago which bears the same name and is the remnant of a volcanic caldera. It forms the southernmost member of the Cyclades group of islands, with an area of approximately 73 kmq.