Unguja as mentioned in The Periplus of the Erythraean Sea is the largest and most populated island of Zanzibar, in Tanzania. Unguja is a hilly island, about 85 km long (north-south) and 30 km wide (east-west) at its widest, with an overall area of about 1666 km². It is located in the southern half of the Zanzibar Archipelago, in the Indian Ocean, about 59 km south of the second largest island of the archipelago, Pemba. Unguja and mainland Tanzania are separated by the Zanzibar Channel.