Right across the street from the Haghia Sophia is the Basilica cistern, begun by Constantine and expanded by Justinian, it was used for storing water for the Imperial Palace complex. It fell into disuse during the Ottoman rule and until recently was the city's dumping ground for trash. It was restored in 1987 after hauling away hundreds of years of trash and is opened to the public. It is quite impressive, in fact, any Byzantine monument is interesting. The unfortunate thing is there isn't much of the Byzantine empire left to see. Why are the two Medusa heads, one on its side, the other upside down? These act as pedestals for the huge columns and were poached from pre-Christian ruins. In a recent show of the 'Bachelorette' they arranged for a romantic dinner here in the cistern.