The rectangular plan measures 138 m × 64.4 m, which puts the storage capacity to c. 80,000 tons of water. It contains 336 columns (all of 9 m height). They are 4.80 m apart from each other, organised in rows of 12 × 28 columns. The cross vaults with round arches between columns consist of square bricks of 38-40 cm width and 4 cm thickness, held together by horasan mortar (Altuğ, 2013, p. 194). Horasan mortar can be found in the Roman, Byzantine, Seljuk and Ottoman periods alike, and was famed for its water resistance (Uğurlu, 2005). The cistern bricks are approximately 6-7 cm; the floor is tiled with baked bricks of 40 × 40 cm. Like other cisterns, the corners are rounded against inward pressure (Altuğ, 2013, p. 194).
A large portion of the capitals is reused, and their styles vary. Most columns are topped by plain basket capitals; 98 carreid acanthus capitals; 3 are reused Corinthian capitals (dating to 200 CE and Constantine?). The Basilica cistern also contains a 4th-century column shaft with a lopped branch design and the inverted Medusa heads (Severan date?) (Crow et al., 2008, p. 138).
This large underground cistern was built by Justinian in the 6th century, and is elevated about 30 m above sea level. It was connected to the Hadrianic line alongside the imperial palace (east of the Hippodrome). See Cod. Just. 11.42.6.
Sometime after the 8-9th century, the overground area of the Basilica started to be used as an artisan quarter after a fire destroyed the octagon in 726 and the university's activities ceased during the Iconoclastic period. During the late middle ages (16th century) and the early modern period, small houses were built on top of the cistern (Müller-Wiener, 1977, p. 284). During the Ottoman period, it was restored several times. During sultan Ahmed III (r. 1703-1730), seven pillars were restored in 1723. Another restoration (details unknown) took place under Abdul Hamid II (r. 1876-1909) (Basilica Cistern, Istanbul Heritage, https://www.basilicacistern.gen.tr/history/).
Forchheimer and Strzygowski, 1893, p. 55 shed more light on the situation at the end of the 19th century. The cistern stairs used to be accessed through a trapdoor opening measuring 95 × 65 cm in the floor of a house at the Şeftali Sk. (according to Forchheimer and Strzygowski belonging to a merchant from Adrianople/Edirne). Apparently the cistern was flooded with water, and without a boat that fit through the opening of the trapdoor, it could only be explored to a limited extent; for the unexplorable parts, the authors relied on Gylles/Gyllius, who explored the cistern with a boat in the 16th century. The authors also report that the cistern was still in use in the 19th century; they reported that the water was not used for drinking as according to Gylles, large fish lived in there.
In 1940 the city government gave the cistern to a museum to administer. Major renovations in 1985-1987 were conducted to prepare the cistern for opening to the public, which happened in 1987. It was restored again by the municipality between 2017-2022.
In the present day, it is a major tourist attraction/museum owned by the Istanbul Metropolitan Municipality. The water level is kept around 1 m. Visitors can explore the cistern on a platform.
