On the nothern side of Divan Yolu is a little park with a stone pillar rising from it. The pillar is a part of an acient aqueduct. Beneath the park , entered by a doorway on its nothern side (On yerebatan caddesi) is the Yerebatan Saray (Sunken Palace) or Cistern Basilica (Tel: +90 212 522 12 59).
Built in 532 AD the Yerebatan Saray or Basilica Cistern  is the largest surviving Byzantine Cistern in Istanbul.
In fact it’s not a basilica at all, but an enormous water storage tank built by Justinian  (527-65) who was incapable of thinking in small terms. Columns capitals and plinths from ruined buildings were among those used in its construction. 
The Cistern also as known as the Sunken Palace is 70 m wide and 140 m long and its roof is supported by 336 coloums. Two coloums in the North western corner  are supported by two blocks carved into Medusa heads. The Cistern was used to support part of the city during lenght sieges.  The water was pumped and delivered through nearly 20 km of aquadect from a resevoir near the Black Sea.
The Cistern  once held 80.000 cubic metres of water but it became a dumping ground of all sort of junks, as well as corpses. Since it was the 18th century and then between 1955 and 1960. The Cistern was then cleaned and renovated between 1985 and 1988 by the Istanbul Municipality.
Today water stil drips through the ceiling and you can see the coloured lights, listen to the western classical music, wander a maze of walkways, and spot carp in the water.
Located in a diagonally across the steet from the AyaSofia.
Yerebatan Saray is open from 9 am to 4.30 pm (5.30 pm in the summer) and admission costs US $ 3.50 US $ 3 for students. The exit from Yerebatan Sarayi is through a gift shop onto Alemdar Street.