Basilica Cistern Medusa heads
Located in the northwest corner of the cistern from Roman empire, the bases of two columns reuse blocks carved with the visage of Medusa.
The origin of the two heads is unknown, though it is thought that the heads were brought to the cistern after being removed from a building of the late Roman period.
There is no written evidence that suggests they were used as column pedestals previously.
Tradition has it that the blocks are oriented sideways and inverted in order to negate the power of the Gorgons' gaze,
however it is widely thought that one was placed sideways only to be the proper size to support the column.
The upside down Medusa was placed that way specifically because she would be the same height right side up.
It is said that Medusa was one of the three underground Gorgona giant sisters.
Out of these three, only Medusa was mortal and she had the power of transforming people who looked at her into stones.
In the old times, the statues and pictures of Medusa were placed in very important buildings and private places to keep them away from bad omens.
In another stories, Medusa is said to be a girl who was very much proud of her black eyes, long hair and beautiful body.
For a long time she was in love with Perseus, the son of Zeus. At the same time, goddess Athene failed in love with him and she became jealous of Medusa.
Athena decided to turn Medusa's beautiful hair into horrible snakes. Then whoever looked at Medusa turned into stone.
When Perseus saw that Medusa was bewitched and she was turning people into stone by a mere glance, he cut her head.
Then he won many wars by showing his beloved one's head to enemies.
From that time onwards it's said that Byzanthine swords' handles were stylised with Medusa's head.
sources: wikipedia.org, own visit and pictures