One of the magnificent ancient buildings of Istanbul is the Basilica Cistern located in the southwest of Hagia Sofia
Constructed for Justinianus I, the Byzantium Emperor (527-565), this big underground water reservoir is called as “Yerebatan Cistern” among the public because of the underground marble columns. As there used to be a basilica in the place of the cistern, it is also called Basilica Cistern.
The cistern is 140 m long, and 70 m wide, and covers a rectangular area as a giant structure. Accessible with 52-step staircase, the Cistern shelters 336 columns, each of which is 9 m high. Erected at 4.80 m intervals from one another the columns are composed of 12 rows, each has 28 columns.
Except couple of the edged and grooved columns of the cistern, majority of them are shaped as a cylinder. Two Medusa heads, which are used as supports under the two columns at the northwest edge of the cistern, are the great work of art from the Roman period. What attracts most attention from the visitors is that the structure from which the Medusa heads have been taken is unknown. The researchers often consider that it has been brought for being used as supports to the column at the time of construction of the cistern. However, this has not prevented myths for the heads of Medusa.
Where is The Basilica Cistern ?
Here is the map you can see it next to Hagia Sophia and Blue Mosque