Santorini, classically Thera, and officially Thira, 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.
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 7 km, is surrounded by 300 m high, steep cliffs on three sides. 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 40 km 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 (sometimes called the Thera eruption), 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.
Firá is the modern capital of Santorini. A traditional settlement, "Firá" derives its name from an alternative pronunciation of "Thíra", the ancient name of the island itself.
Fira is a city of white-washed houses built on the edge of the 400 metres high caldera on the western edge of the semi-circular island of Thera. Access to Fira is mainly by roads on its eastern side, climbing from its port via the Z-shaped footpath on foot or on donkeys, or by riding the steep cable car from its lower terminal by the port.
From Fira there is a panoramic view of the 18 kilometres long caldera from southern Cape Akrotiri to northern Cape Ag. Nikolaos, plus the volcanic island Nea Kameni at the center with Thirassia Island in the Aegean.