A visit to the eerily beautiful underground football field size cistern is a favorite on our Istanbul private guided tour. It's easy to walk to as it's located in the Sultanahmet district in the center of the city. You can walk to the Blue Mosque and Hagia Sophia on the same day.
The waters came from 19 kilometers away and the cistern could hold 100,000 tons of water!
After walking down the 52 stairs, you'll be in this present day 420' x 210' huge cistern, with it's 336 marble pillars towering at 27'. Taking the walkway you'll see the fish filled waters and ancient heads of Medusa. Emperor Justinian I built it in 532 to provide water to the grand palace. Later the waters were used to irrigate the gardens of the Topkapi Palace. Now, it is a museum and open to visitors and concerts are held here as well.
Other helpful links: Cultural tips:
Turkish phrases for your first trip.http://bit.ly/1fykfzw
FYI: Not mentioned in the book is the fact that you can pose in old traditional Sultan costumes before you walk around the cistern area which is a great souvenir.