According to http://www.istanbultrails.com/2008/06/the-basilica-cistern-the-coolest-spot-in-town/

The Basilica Cistern, which borrowed its name from the Ilius Basilica, is 143 meters long and 65 meters wide. The roof is supported by 336 marble columns, mostly in Ionic or Corinthian styles, each measuring 9 meters in length. Spaced at four-meter intervals, they are arranged in 12 rows of 28 columns each.

The cistern could hold 80.000 cubic meters of water, coming from the Eğrikapı Water Distribution Centre in the Belgrade Forest, 19 kilometers from the city. The water was transported to the city center via the 971-meter-long Valens Aqueduct (Bozdoğan Sukemeri) and the 11.545-meter-long Mağlova Aqueduct (Mağlova Sukemeri), which was built by Emperor Justinian I.

The cistern was forgotten for centuries and only accidently rediscovered by the Frenchman Peter Gyllius in 1545. While researching Byzantine antiquities in the city, he noticed that people in the neighborhood not only got water by simply lowering buckets through holes in their basements, they miraculously sometimes even caught fish this way.