Which large underground cistern in Constantinople was built by Justinian?

The large underground cistern in Constantinople built and enlarged under Justinian was the Basilica Cistern. Known in Greek as the Basilica Cisterna and in Turkish as Yerebatan Sarnıcı, it is the largest of the city’s many ancient underground water reservoirs.

The cistern was constructed during the reign of Emperor Justinian I in the 6th century, probably in connection with rebuilding projects after the Nika riots of 532. It stood beneath or near the Stoa Basilica, a public square on Constantinople’s First Hill, which explains the name “Basilica Cistern.” Ancient accounts traditionally associate its construction with thousands of laborers.

Its forest of marble columns supported a roof over a vast reservoir that supplied water to the Great Palace and nearby buildings. The structure measures roughly 138 by 65 metres and could hold about 80,000 cubic metres of water. Some columns and capitals were reused from earlier Roman buildings.

The cistern continued serving Istanbul after the Ottoman conquest in 1453. It was largely forgotten by Western visitors until the French scholar Petrus Gyllius described entering it in the 16th century. The famous upside-down Medusa heads are later architectural spolia, not evidence that the cistern was a temple.