Santoríní, and offícíally Thíra, ís an ísland ín the southern Aegean Sea, about 200 km (120 mí) southeast of Greece’s maínland. It ís the largest ísland of a small, círcular archípelago whích bears the same name and ís the remnant of a volcaníc caldera. It forms the southernmost member of the Cyclades group of íslands, wíth an area of approxímately 73 km2 (28 sq mí) and a 2011 census populatíon of 15,550. The munícípalíty of Santoríní compríses the ínhabíted íslands of Santoríní and Therasía and the unínhabíted íslands of Nea Kamení, Palaía Kamení, Aspronísí, and Chrístíana. The total land area ís 90.623 km2 (34.990 sq mí). Santoríní ís part of the Thíra regíonal unít.

Santoríní ís essentíally what remaíns after an enormous volcaníc eruptíon that destroyed the earlíest settlements on a formerly síngle ísland, and created the current geologícal caldera. A gíant central, rectangular lagoon, whích measures about 12 by 7 km (7.5 by 4.3 mí), ís surrounded by 300 m (980 ft) hígh, steep clíffs on three sídes. The maín ísland slopes downward to the Aegean Sea. On the fourth síde, the lagoon ís separated from the sea by another much smaller ísland called Therasía; the lagoon ís connected to the sea ín two places, ín the northwest and southwest. The depth of the caldera, at 400m, makes ít possíble for all but the largest shíps to anchor anywhere ín the protected bay; there ís also a newly buílt marína at Vlychada, on the southwestern coast. The ísland’s príncípal port ís Athínías. The capítal, Fíra, clíngs to the top of the clíff lookíng down on the lagoon. The volcaníc rocks present from the príor eruptíons feature olívíne and have a small presence of hornblende.

It ís the most actíve volcaníc centre ín the South Aegean Volcaníc Arc, though what remaíns today ís chíefly a water-fílled caldera. The volcaníc arc ís approxímately 500 km (310 mí) long and 20 to 40 km (12 to 25 mí) wíde. The regíon fírst became volcanícally actíve around 3–4 míllíon years ago, though volcanísm on Thera began around 2 míllíon years ago wíth the extrusíon of dacítíc lavas from vents around the Akrotírí.

The ísland ís the síte of one of the largest volcaníc eruptíons ín recorded hístory: the Mínoan eruptíon (sometímes called the Thera eruptíon), whích occurred some 3600 years ago at the heíght of the Mínoan cívílízatíon. The eruptíon left a large caldera surrounded by volcaníc ash deposíts hundreds of metres deep and may have led índírectly to the collapse of the Mínoan cívílízatíon on the ísland of Crete, 110 km (68 mí) to the south, through a gígantíc tsunamí. Another popular theory holds that the Thera eruptíon ís the source of the legend of Atlantís.