Santorini is the remnant of a volcanic caldera located in the Mediterranean.
The Aegean Sea ( [ ᵻˈdʒiːən ] Αιγαίο Πέλαγος [ eˈʝeo ˈpelaɣos ] ; Ege Denizi [ eɟe denizi ] ) is an elongated embayment of the Mediterranean Sea located between the Greek and Anatolian peninsulas , i.e. , between the mainlands of Greece and Turkey . 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 . It is the most active volcanic centre in the South Aegean Volcanic Arc , though what remains today is chiefly a water-filled caldera .