The Tonga Islands (Pule'anga Tonga), are located in the South Pacific (Polynesia) between the Fiji, Samoa and the Cook Islands, is a constitutional monarchy associated with the British Commonwealth. The state is formed by the archipelagos of Tongatapu, Vava'u and Ha'apai along with some scattered islands (Niuatoputapu, Niuafo'ou and Tafahi) for a total of about 150 islands, the islands emerging from one of the deepest oceanic trenches, Tonga Trench (-10 882 metres). The capital Nuku'alofa is on the island of Tongatapu (260 sq km), which is the largest and the most densely populated island, other important islands are Vava'u, 'Eau, Lifuka and Foa. Most of the islands are flat coral islands of origin, mostly atolls, but some are high volcanic islands with active volcanoes, as the islands of Kao (1,030 metres), Tafahi (560 metres), Niuafo'ou (260 m), Late (540 metres) and Tofua (515 metres), many are also the underwater volcanoes like Curacoa (whose top is located only 33 meters deep), Home Reef (which emerged in 2006), Metis Shoal and Fonualei. Agriculture is the basis of the island economy there are cultivated cassava, bananas, coconuts, sweet potatoes, citrus fruits, and vanilla, are also very important fisheries and livestock (pigs, cattle, horses). Tourism is a good source of income.