Basilica Cistern*4

On the side where the steps are, there is actually a tank with a lot of fish swimming in it. I didn't realise the cistern had a whole aquatic ecosystem but apparently it does.

Despite all the myths and tales of how Medusa's gaze turns people to stone and that being the reason the column bases were rotated to their sides (as well as upside down), it is more likely that they were positioned this way to be the correct height and size to support the column above it.

You probably wouldn't know it unless you're quite elderly, but the Cistern was used in the Bond film "From Russia with Love" in the 60's. In that film, it was in-accurately described as being constructed by the Emperor Constantine but we now know (and perhaps they also knew too) that it was constructed by Justinian.