Cistern

The emperor Justinian built a huge underground cistern, in the 7th century, near the Hippodrome, to provide water to the city in times of siege. Over the centuries it was neglected, fell into disuse, and eventually was forgotten. Rediscovered, renovated, it is now a major sight, and an impressive one too.

The whole thing is 20-30 feet underground, now only partially flooded, with a 
boardwalk for visitors, attractively lighted; in area, about the size of two 
football fields

One of the columns; the whole thing was built using spare/
disused parts, so hardly any columns or capitals match

One of the two Medusa heads; capitals, one upside down...

The other, sideways

View of another part of the Cistern