Zanzibar is part of the East African republic of Tanzania. It consists of the Zanzibar Archipelago in the Indian Ocean, 25-50 km (15-30 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.