The Cisterns of Tawila, or the Tawila Tanks, is a historic site in Aden, Yemen designed to collect and store the rain that flows down from the Shamsan massif through Wadi Tawila, and to protect the city from periodic flooding. The site consists of a series of tanks of varying shape and capacity. They are connected to one another and located in Wadi Tawila to the southwest of Aden’s oldest district, Crater. Originally there were about 53 tanks, but only 13 remain following a succession of renovations, including those done by the British in the 19th century. The existing tanks have a combined capacity of about nineteen million gallons. The largest of the tanks are the Coghlan Tank at the center of the main site and the large, circular Playfair Tank, located at the lowest point, outside the main site.
Ream more about Cisterns of Tawila : https://en.wikipedia.org/wiki/Cisterns_of_Tawila