The Basilica Cistern was built in the sixth century to hold water for the Great Palace, and it has been holding it, in one form or another, ever since. Three hundred and thirty-six columns stand in rows under a low brick vault, most of them taken from older Roman buildings, which is why no two capitals quite match.

Two of the columns rest on carved Medusa heads, one upside down and one on its side. Nobody knows why they were placed that way, and the guesses have been accumulating for fifteen hundred years.
It stays cool inside even in August, which makes it a good midday stop. From Hotel Sultania it is a walk of about ten minutes, and the entrance sits almost opposite Hagia Sophia, so the two are easily combined.