The Basilica Cistern (Βασιλική Κινστέρνα) is the largest underground cistern of Byzantine Constantinople. It was built under the Basilica located west of Hagia Sophia. Also known by its Turkish name Yerebatan Sarayı (“Sunken Palace”), it is one of the major tourist attractions of modern Istanbul. The Basilica Cistern, along with churches like Hagia Sophia, gives an impression of the massive construction projects once undertaken by Justinian I (527-565).

The Basilica Cistern is the only remaining structure of an important complex of buildings in the vicinity during Late Antique Constantinople. The cistern gets its name from the Basilica, often known as the Basilica Stoa or the Basilica of Illus, once located above it. Nothing is known about the site before Byzantium was inaugurated as Constantine’s capital. It was located west of the Augustaion and Hagia Sophia, and northwest of the Milion, the milestone at the beginning of the Mese. The Basilica, where Constantinople’s law courts were situated, was erected around the late 4th or early 5th century, with a large courtyard surrounded by four porticoes. Buildings attached to the Basilica included, the Library of Constantinople (said to contain 120,000 books in the 5th century) and the Octagon (the seat of a university where law courses were given and cases tried). Various stalls, including book sellers, were also located in the Basilica. The Basilica burned down in 476, after which it was rebuilt probably by Illus. It was again destroyed by fire in the Nika Riot of 532 during the reign of Justinian. The basilica was rebuilt by the prefect Longinus by 542, during which time the Basilica Cistern was built underneath it. Work probably began on the cistern in 527–28 and probably functioned some years before the Basilica was finished in 541. During the reign of Justin II a horologion, which was perhaps a sundial, was set up at the Basilica. The Octagon was reopened by Patriarch Sergius after being closed by Phocas (602-610), though it is permanently closed during Iconoclasm. It seems that the Basilica later served as quarters for craftsmen. Travelers to Constantinople, including Pierre Gilles in the 16th century, give accounts of the cistern. The cistern underwent repairs on several occasions during the Ottoman era. A section on its northwestern side was walled off, while eight columns on its eastern side were encased in concrete during construction work in the 1950s. In 1940, the cistern was handed over to the museum administration in order to prepare it for tourists.

The Basilica has a rectangular plan with beveled corners, measuring around 138 x 65 meters. Its capacity was around 80,000 cubic meters. Its vaults, which consist of bricks arranged in concentric squares up to its crown, are supported by 336 columns (12 rows of 28 columns that are around 9 meters high). The columns are around 9 meters tall and spaced 4.8 meters apart. It has plain impost capitals, as well 98 Corinthian capitals, which were perhaps in storage. Many of its shafts and plinths seem to have been reused. Its walls are lined with hydraulic plaster. The cistern was likely supplied by the Aqueduct of Hadrian which was restored by Justinian in 528. Procopius indicates that the cistern was built for the water shortages occurring during the summer (which is the dry season for the region). It, then, played a similar role to Binbirdirek Cistern, which was supplied by the Aqueduct of Valens.  It is unclear whether the cistern provided water to the public in the area or only to the nearby palace complex. While there were already massive open-air cisterns, this cistern probably provided cleaner water than the older open-air cisterns.

 The most notable examples of reused materials are the two columns decorated with Gorgon heads and the so-called “peacock-eyed” or “tear-drop” column. A similar Gorgon head, now at the Istanbul Archaeological Museums, was found at the Forum of Constantine, which suggests the two Gorgon heads of the Basilica Cistern might also originally decorated the Forum of Constantine.  The “peacock-eyed” column, which is similar to column fragments from the Forum of Theodosius, resembles the artistic convention of the Club of Hercules, as seen in the famous Farnese Hercules, which depicts Hercules resting on his club. This would mean that the column is decorated with knots of a tree, rather than peacock eyes or tear-drops. While the Gorgon head in particular have often been the source of speculation, it is unclear if the reused material had any particular significance.