Beneath Istanbul lie hundreds of gloomy Byzantine cisterns. They
have been there since the days of Constantinople. The grandest of all is
Yerebatan Saray Sarnici, called the Sunken Palace because of its size
(70 x 140 meters, or 2.4 acres), its capacity (80,000 cubic meters--over
21 million US gallons) and its 336 marble columns. Some of the columns
have been reused from roman remains, see the heads in these two photographs.
Walkways and atmospheric lighting were installed during the 1990s
so you can see all its curious corners. Unfortunately they do have a tendency
to make everything look red when not using a flash.