First, the main attraction:
It is actually impossible to capture the scale of the building.
It was a church for over 900 years.
Then it was a mosque for almost 500 years.
Not many of the mosaics have survived the centuries, but there are a few:
There are signs everywhere to not use flash when taking pictures, but the windows are open. I'm not sure how much longer the mosaics can survive Istanbul air (not to mention pigeons flying around).
There's lots of marble.
Ataturk made it into a museum, so it is no longer a functioning mosque; that's why the mosaics can be uncovered.
Don't know if you've read Inferno by Dan Brown, but that little adventure takes you into the Cistern. Photos are pretty hard to take underground, but I did snap shots of the two Medusas. I read the novel as part of my prep work before coming to Istanbul :-D
The Little Hagia Sophia is close to my new (modest) hotel. I hadn't realized it is a functioning mosque until I got there. Luckily I had my headscarf with me (don't leave home without it).
I wasn't sure they would let me in with my ankles showing ... but they did.