The Yerebatan Cistern, or "Sunken Cistern", is the largest underground cistern in Constantinople. Originally used as a basilica (hence its alternate epithet, the Basilica Cistern) this once grand public space - with its many hundreds of marble columns - was converted in the sixth century CE into a water storage facility for the Byzantine palace complex situated immediately to the east.
After the fall of the Latin dynasty and the restoration of the Palaiologi, the cistern fell out of use and was forgotten by all but a few residents of the city. By the time the Ottomans had taken over in 1453, all common knowledge of them had been lost.
Still, rumors of the cistern's existence persisted in the minds of the imaginative and the hopeful, bolstered by the repeated stories of men and women who claimed to have active fishing holes in basements of their houses. It wasn't until late in the reign of Suleiman the Magnificent that a visting diplomat rediscovered the cistern's entrance and passed on his findings to a most amused and fascinated Sultan.