The Basilica Cistern (Turkish: Yerebatan Sarayı - "Sunken Palace", or Yerebatan Sarnıcı - "Sunken Cistern") is located in opposite of Sultan Ahmed Mosque (Blue Mosque). it's a nice, mysterious place, in which you feel, you are in old epochs. Also there is a stand, in which you can take some photos with different kinds of old dresses of Ottomons. it's too intersting. when you are in Istanbul, one of the most important sight seeings you should visit, may be underground cistern.
It is an amazing Roman building made 532 AD. There are lots of columns. There are different column heads because romans recycled that columns from Pagan temples. Most of the travel agencies don't put underground cistern in their program.

Entrance fee is 20 TL (as of June 2014), Turkish citizens 5 TL. Open every day from 9 am until 6:30 pm.