This is the Basilica Cistern, just a few steps away from the Hagia
Sofia. Emperor Justinian had this place built in 532 to meet the
increasing water needs of the growing city. It is a fantastic place. I
visited it twice. You walk around on elevated walkways amidst
dimly lit columns. They play soft, moody, oud music, which blends in
with the dripping of water.
Tucked back in the corner, two columns are supported on Medusa head
bases. These are most likely plunder from earlier monuments.
Do not park illegally in Istanbul. They have squadrons of these crane
trucks that wll show up within 3 nanoseconds of any parking infraction,
squawk in loud Turkish for 30 seconds and then "whoosh!" off goes your
car to fates too horrible to contemplate.