The Basilica Cistern

The Basilica Cistern is a large underground water reservoir in Istanbul, very close to the Hagia Sofia and the Blue Mosque.  Constructed about 600 BC by Romans, it held a large quantity of water for the city, carried there by aqueducts from the Belgrade Woods 19 kilometres away.

The upside-down Medusa's head.

Today it is just a tourist attraction, and the water is about half a metre deep.  There is not a lot to say about the cistern except that it is surprisingly large (140m by 90m by 9m high), and two of the 336 marble columns are sitting on big blocks carved with Medusa heads.  Apparently this was just expediency, with builders often re-using blocks from older dismantled structures.

It is nice and cool down there, so it is an excellent respite from a hot day in Istanbul.