Santorini, officially Thira , is an island located in the southern Aegean Sea, about 200 kilometer (120 mi) southeast from 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 square kilometer (28 sq mi) and a 2001 census population of 13,670. The municipality of Santorini comprises 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 square kilometer (34.990 sq mi). Santorini is part of the Santorini peripheral unit. Santorini is essentially what remains after an enormous volcanic explosion 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 kilometer (7.5 by 4.3 mi), is surrounded by 300 meter (980 ft) 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 water in the center of the lagoon is nearly 400 meter (1,300 ft) deep, thus making it a safe harbor for all kinds of shipping. The island's harbors all lie in the lagoon, and there are no ports on the perimeter of the island; 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 notably small presence of hornblende.