Aden consists of a number of distinct sub-centers: Crater, the original port city; Ma'alla, the modern port; Tawahi, known as "Steamer Point" in colonial days; 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--the former RAF Khormaksar. On the mainland are the sub-centers of Sheikh Othman, a former oasis area; Al-Mansura, a town planned by the British; and Madinat ash-Sha'b (formerly Madinat al-Itihad), 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 harbor that comprises the modern port. The volcanic peninsula of Little Aden forms a near-mirror image, enclosing the harbor 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.