Istanbul lie hundreds of gloomy Byzantine cisterns. They're left from the days when Istanbul was Constantinople.
The grandest of all is the Basilica Cistern (Yerebatan Sarnıç), so called because it lay beneath the Stoa Basilica, a grand Byzantine public square. It's also called the Sunken Palace Cistern (Yerebatan Saray Sarnıçı) because that's what it looks like.
Whatever you call it, it's impressive because of its size, measuring 138 meters long and 64.6 meters wide, covering nearly 1000 square meters (2.4 acres); its capacity (80,000 cubic meters—over 21 million US gallons) and its 336 marble columns.
Remember the scene in the old James Bond movie From Russia With Love when Bond is rowing in a small boat through a forest of marble columns? That scene was filmed in Yerebatan.
Built by Justinian after 532, the Basilica Cistern stored water for the Great Palace and nearby buildings. Lost to memory, it was rediscovered by Petrus Gyllius, who came to Constantinople in search of Byzantine monuments. Gyllius, who noticed that local people got their water by lowering buckets through holes in the floors of their houses, found an entrance and thus put it back on the map.
The Ottomans used it to supply Topkapı Palace.