The Basilica Cistern and its Medusas

This tiny building is just the way in. Beneath your feet stretches a hall 138 metres long and 65 wide, with 336 columns rising out of the water in the half-light. The Turks call it Yerebatan Sarayı, "the sunken palace".

1Water for the palace

Justinian had it built in the 6th century beneath a large public basilica, hence its name. It held some eighty thousand cubic metres of water, brought in by aqueduct to supply the Great Palace and the buildings around it. Many of the columns were reused from older temples, which is why no two are alike.

2The traveller who fished in the dark

After the conquest, the cistern was forgotten. In the mid-16th century the French scholar Pierre Gilles discovered that local residents drew up water, and even fish, through holes in the floors of their houses. He managed to get down and explored it by boat, among columns that vanished into the darkness.

3The two Medusas

In the north-west corner, two columns rest on blocks carved with Medusa's face: one on its side and one upside down. Nobody knows where they came from.

Tradition says they were placed that way to neutralise her gaze, which turned people to stone; most likely the builders simply needed blocks of the right height.

Another column is covered in carved teardrops. They're said to weep for the slaves who died building the cistern, though the pattern is a known decorative motif called the "hen's eye".

4What to look for

• The entrance building, tiny compared with what it hides.
• The Milion, right opposite, across the street.
• If you go down, the two Medusa heads and the column of tears.