The Basilica Cistern was the Byzantine underground water reservoir in Constantinople commissioned by Emperor Justinian I.
Known in Turkish as Yerebatan Sarayı, or “Underground Palace,” the cistern was built in the sixth century during Justinian’s reign. It measures roughly 138 by 65 metres and was supported by hundreds of marble columns arranged in regular rows. Its enormous capacity helped supply water to the imperial city, especially the Great Palace and nearby buildings.
The cistern’s name comes from the Basilica of Illus, a former public building that stood above it; it was not itself a basilica-shaped church. Visitors often remember its two reused Medusa column bases, which are ancient spolia rather than purpose-made Byzantine decorations.
The structure was restored and opened to visitors in the twentieth century. It remains one of Istanbul’s best-preserved examples of Byzantine hydraulic engineering and illustrates how Constantinople’s architecture depended on extensive water-storage systems.