Aden is a seaport city in Yemen, located by the eastern approach to the Red Sea, some 110 miles east of Bab-el-Mandeb. Its population is approximately a million people. Aden's ancient, natural harbour lies in the crater of a dormant volcano which now forms a peninsula, joined to the mainland by a low isthmus. This harbour, Front Bay, was first used by the ancient Kingdom of Awsan between the 5th and 7th centuries BC. The modern harbour is on the other side of the peninsula.Aden consists of a number of distinct sub-centres: Crater, the original port city; Ma'alla, the modern port; Tawahi, known as "Steamer Point" in the colonial period; and the resorts of Gold Mohur. Khormaksar, located on the isthmus that connects Aden proper with the mainland, includes the city's diplomatic missions, the main offices of Aden University, and Aden International Airport, Yemen's second biggest airport. On the mainland are the sub-centres of Sheikh Othman, a former oasis area; Al-Mansura, a town planned by the British; and Madinat ash-Sha'b, the site designated as the capital of the South Arabian Federation and now home to a large power/desalinization facility and additional faculties of Aden University.Aden encloses the eastern side of a vast, natural harbour that comprises the modern port. The volcanic peninsula of Little Aden forms a near-mirror image, enclosing the harbour and port on the western side. Little Aden became the site of the oil refinery and tanker port. Both were established and operated by British Petroleum until they were turned over to Yemeni government ownership and control in 1977.Aden was the capital of the People's Democratic Republic of Yemen until that country's unification with the Yemen Arab Republic. On that occasion, the city was declared a free trade zone. Aden gives its name to the Gulf of Aden.