The Basillica Cistern

 This underground water system  was used for the city of Constantinople and then later for the Ottoman Empire.

The water system sits underneath the city of Istanbul.  Many columns allow the cistern to remain under the city without it collapsing.  Two of the columns have Medusa heads as the base of the column.  The Medusa head dates back to Roman times. 

If you compare these columns to the top picture, you will notice that the shape of the columns changes.