One of the most delightful hidden treasures in Istanbul is the Basilica Cistern, an underground reservoir in the vicinity of Hagia Sophia. Often called the Great Cistern of Justinian, the Basilica Cistern is no ordinary cistern. The Turkish refer to it as Yerebatan Sarayi or the Sunken Palace, and when you step down into the underground chamber it does look like a sunken palace. Read more about the Basilica Cistern Here.