Zanzibar is an insular autonomous region of Tanzania. It is composed of the Zanzibar Archipelago in the Indian Ocean, 25–50 kilometers (16–31 mi) off the coast of the mainland, and consists of many small islands and two large ones: Unguja (the main island, referred to informally as Zanzibar) and Pemba Island. The capital is Zanzibar City, located on the island of Unguja. Its historic center is Stone Town, a World Heritage Site. In April 1964, the People’s Republic of Zanzibar and Pemba merged with mainland Tanganyika. This United Republic of Tanganyika and Zanzibar was soon renamed, blending the two names, as the United Republic of Tanzania, within which Zanzibar remains an autonomous region.