Zanzibar (pronounced /ˈzænzɨbɑr/) is part of the East African republic of Tanzania. It consists of the Zanzibar Archipelago in the Indian Ocean, 2550 km (1530 mi) off the coast of the mainland. There are numerous small islands and two large ones: Unguja (the main island, informally referred to as "Zanzibar"), and Pemba. Zanzibar was once a separate state with a long trading history within the Arab world; it united with Tanganyika to form Tanzania in 1964, and still enjoys a high degree of autonomy within the union. The capital of Zanzibar, located on the island of Unguja, is Zanzibar City, and its old quarter, known as Stone Town, is a World Heritage Site.