Thera (now Santorin), an island of the AEgean sea, now forming with Amorgos and other islands an eparchy of Greece, in the no-marchy of the Cyclades; length about 9 m. from N. to 8., average breadth about 4 m.; pop. about 13,000; of the eparchy, in 1870, 21,907. It was originally circular, but the islet Therasia was torn from it by an earthquake about 237 B. C, and it now resembles a horseshoe. The harbor thus formed is the crater of a volcano, and as no bottom is found, vessels make fast to the abrupt and rocky shores. The soil is volcanic and inclined to dryness, but very fertile. The annual production of wine is about 1,750,000 gallons. Ship building is the only considerable industry. Thera, the capital, had a population in 1870 of 5,143.- Though an ancient Lacedaemonian colony, Thera is only of historic importance as having sent a colony to found the city of Cyrene in Africa, 631 B. C. The dates of the eruptions known to have taken place in or near this island are 197 B. C. .and A. D. 46, 726, 1573, 1707, and 1866. By that of 197 B. C. the island of Palaea (Old) Cammeni was formed, by that of 46 Mikra (Little) Cammeni, and by that of A. D. 1707 Nea (New) Cammeni. The last was at first composed of white pumice, but subsequently received additions of brown trachytic rock.

The eruption did not wholly cease or the island assume its present form till 1712. In the beginning of 1866 stones flew up from the port of Volcano, and a new volcano arose which attained a height of about 100 ft. The eruptions continued until the autumn of 1870, and enormous quantities of lava were thrown out, surpassing in size those projected in 1707-'12. Near Nea Cammeni a regular cone was formed 325 ft. high.