Basilica Cistern Sultanahmet

Ah, to control the water supply of Constantinople ... Water brought in was stored under this church, also featured in a scene in From Russia With Love in 1963.

 Two of the columns in the cistern have the head of Medusa at the base of the column.  One is right side up, one is on its side.  Nobody seems to know why these two are placed where they are, and they are the only ones like them among 300+ other largely ionic and doric columns.