Claim: Santorini is in a sea.

Santorini -LRB- Σαντορίνη , pronounced -LSB- sandoˈrini -RSB- -RRB- , classically Thera -LRB- English pronunciation -LSB- ˈθɪərə -RSB- -RRB- , and officially Thira -LRB- Greek : Θήρα -LSB- ˈθira -RSB- -RRB- , 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 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 . The municipality of Santorini includes the inhabited islands of Santorini and Therasia and the uninhabited islands of Nea Kameni , Palaia Kameni , Aspronisi , and Christiana . The total land area is 90.623 km2 . Santorini is part of the Thira regional unit .   Santorini is essentially what remains after an enormous volcanic eruption that destroyed the earliest settlements on a formerly single island , and created the current geological caldera . A giant central , rectangular lagoon , which measures about 12 by , is surrounded by 300 m high , steep cliffs on three sides . The main island slopes downward to the Aegean Sea . On the fourth side , the lagoon is separated from the sea by another much smaller island called Therasia ; the lagoon is connected to the sea in two places , in the northwest and southwest . The depth of the caldera , at 400m , makes it impossible for any but the largest ships to anchor anywhere in the protected bay ; there is also a fisherman 's harbour at Vlychada , on the southwestern coast . The island 's principal port is Athinios . The capital , Fira , clings to the top of the cliff looking down on the lagoon . The volcanic rocks present from the prior eruptions feature olivine , and have a small presence of hornblende .   It is the most active volcanic centre in the South Aegean Volcanic Arc , though what remains today is chiefly a water-filled caldera . The volcanic arc is approximately 500 km long and 20 to wide . The region first became volcanically active around 3 -- 4 million years ago , though volcanism on Thera began around 2 million years ago with the extrusion of dacitic lavas from vents around the Akrotiri .   The island is the site of one of the largest volcanic eruptions in recorded history : the Minoan eruption -LRB- sometimes called the Thera eruption -RRB- , which occurred some 3,600 years ago at the height of the Minoan civilization . The eruption left a large caldera surrounded by volcanic ash deposits hundreds of metres deep and may have led indirectly to the collapse of the Minoan civilization on the island of Crete , 110 km to the south , through a gigantic tsunami . Another popular theory holds that the Thera eruption is the source of the legend of Atlantis .