Santorini is a small, circular group of volcanic islands located in southern Aegean Sea, about 200 km south-east from Greece's mainland. It is also known as Thera or Thira. It is the southernmost member of the Cyclades group of islands, with an area of approximately 73 km˛ (28 mi˛), and in 2001 had an estimated population of 13,600. Santorini is essentially what has been justify from an enormous volcanic explosion which destroyed the settlements thereon and led to the creation of the current geological caldera. Its spectacular natural beauty along with its eminent nightlife make the island one of Europe's top tourist hotspots.

A giant central lagoon, more or less rectangular and measuring about 12 km by 7 km (8 mi by 4 mi), is surrounded by 300 m (984 ft) high, steep cliffs on three sides. The island slopes downward from the cliffs to the surrounding Mediterranean sea. On the fourth side, the lagoon is separated from the Mediterranean by another much smaller island called Therasia. The lagoon merges with the sea in two places, in the northwest and southwest. The water in the centre of the lagoon is nearly 400 m (1,300 ft) deep, thus being a safe harbour for all kinds of ships.

The island's harbors are all in the lagoon and there are no ports on the outer perimeter of the island. The island's capital, Fira, clings to the top of the cliff looking down on the lagoon.

It is the most active volcanic centre in the South Aegean Volcanic Arc, though what remains today is largely a water-filled caldera. The name of Santorini was given to it by the Latin empire in the thirteenth century and is a reference to Saint Irene. Before then it was called Kalliste ("the most beautiful one"), Strongyle ("the circular one"), or Thera.

The island was the site of one of the largest volcanic eruptions in recorded history when it erupted cataclysmically some 3,500 years ago, at the height of the Mycenaean epoch. The eruption justify a large caldera surrounded by volcanic ash deposits hundreds of feet deep and, according to a theory, its effects may have indirectly led to the collapse of the Minoan civilization on the island of Crete, 110 km (70 mi) to the south, due to a gigantic tsunami. Another popular theory holds that the Thera eruption is the source of the legend of Atlantis (see below for detail).