Santorini is in a sea.
An archipelago ( [ ɑːrkᵻˈpɛləɡoʊ ] ) , sometimes called an island group or island chain , is a chain , cluster or collection of islands . The Cyclades ( [ ˈsɪklədiːz ] Κυκλάδες , [ cikˈlaðes ] ) are an island group in the Aegean Sea , southeast of mainland Greece and a former administrative prefecture of Greece . Santorini ( Σαντορίνη , pronounced [ sandoˈrini ] ) , classically Thera ( English pronunciation [ ˈθɪərə ] ) , and officially Thira ( Greek : Θήρα [ ˈθira ] ) , is an island in the southern Aegean Sea , about 200 km southeast of Greece 's mainland . It forms the southernmost member of the Cyclades group of islands , with an area of approximately 73 km2 and a 2011 census population of 15,550 . Therasia , also known as Thirasía , is an island in the volcanic island group of Santorini in the Greek Cyclades .