The Engineering of a Water Reservoir

To understand the function of the Basilica Cistern, we first need to know how water reached it. In Constantinople, water was carried from sources outside the city through long channels and aqueducts. The main force that moved the water through this system was gravity. The channels were built with a very gentle slope. This allowed water to flow from higher points to lower ones without the need for pumps. To achieve this, however, the gradient had to be calculated precisely over long distances. Historical sources indicate that the Basilica Cistern was supplied mainly by the Hadrian water line. Water arriving in the city could be stored here until it was needed. The cistern is approximately 140 meters long and 65 meters wide. It had a storage capacity of around 80,000 cubic meters, in other words, 80 million liters of water. Storing such a vast quantity of water posed a serious engineering challenge. First of all, the walls had to withstand the water pressure. The cistern's brick walls are approximately four meters and eighty centimeters thick. Another challenge was preventing the water from leaking. The walls and floor were coated with Khorasan mortar, which contains crushed brick and lime-based materials. This coating helped make the structure watertight. Finally, the weight bearing down on this vast underground space had to be supported. Brick arches and vaults transferred this load downward through the columns. This was the essential role of the columns and arches you see today: to ensure that a large water reservoir was both sturdy and usable. Now let's take a closer look at the most striking elements of this load-bearing system, the marble columns.