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 . The municipality of Santorini includes the inhabited islands of Santorini and Therasia and the uninhabited islands of Nea Kameni , Palaia Kameni , Aspronisi , and Christiana . Therasia , also known as Thirasía , is an island in the volcanic island group of Santorini in the Greek Cyclades . It lies north-west of Nea Kameni , a small island formed in recent centuries by volcanic activity and thus marking the centre of the island group .