Therasia is outside of the volcanic island group of Santorini.
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 is the largest island of a small , circular archipelago which bears the same name and is the remnant of a volcanic caldera . Therasia , also known as Thirasía , is an island in the volcanic island group of Santorini in the Greek Cyclades .