The Basilica Cistern, located in the crowded Eminönü district of Istanbul next to the Hagia Sophia, was built to provide water for the city of Istanbul during the reign of Emperor Justinian I in the 6th century CE. This cistern is an underground chamber of 138 x 64.6 metres. The large space is broken up by a forest of 336 marble columns, which are aesthetically supported by strong columns and arches. The ceiling vaults, known as Manastır Tonozu (cloister vault), are built without using a mould. The cistern is surrounded by a firebrick wall with a thickness of 3.5 meters and is coated with a special mortar to make it waterproof.