Aden encloses the eastern side of a vast, natural harbour that comprises the modern port. A long time ago this necessitated the existence of Aden's reservoirs, the Cisterns of Tawila. As described by 14th century scholar Ibn Battuta, "These reservoirs accumulate rainwater for the sole purpose of drinking for the city's citizens. The city is prosperous with rich merchants living here and Indian vessels arriving for trade." The volcanic peninsula of  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 1978.