There is a large open-air cistern once located north of the Hebdomon, a coastal suburb of Constantinople located on the Via Egnatia. Unusual in being located outside the Theodosian Walls, it possibly supplied the Hebdomon, which was  also the site of an army camp and an imperial palace. Its Turkish name Fildamı “Elephant Stables” derives from a tradition that the elephants from a nearby Ottoman palace were kept here. It was later used as a “sunken garden” (Çukurbostan) during the Ottoman era as the open-air cisterns within the city walls. It is now a park in the district of Bakırköy in Istanbul.

The cistern, which perhaps dates to the 6th or 7th century, is around 127 x 76 meters, with a maximum capacity of roughly 125,000 cubic meters. Its walls are faced with alternating bands of brick and ashlar, similar to the Theodosian Walls and other open-air cisterns. The interior of the walls originally measured 11 meters, though its floor is now covered with soil. It is built on a sloping hill, with its western wall built against the rising hill, while its eastern wall is uncovered. Both the outer face of the east wall and the inner face of the west wall are reinforced by a series of niches measuring around 7 meters thick. The northern and southern walls, which once had a double staircase on the inside, are around 4 meters thick. At a later date, a water tower was added near its southeastern corner to help regulate water flow.