"An Underground Wonder A city as ancient as Istanbul has many layers of history, and you’ll have to go below the surface to see the Basilica Cistern, the largest of the underground cisterns. Built in the sixth century for the Byzantine emperor Justinian, the atmospheric reservoir is supported by 336 columns, many of which have been repurposed from other ruins over the centuries. The Medusa-head column bases are a favorite to photograph, along with the carp that swim silently in the dimly lit waters. The cistern is located between the Blue Mosque and Hagia Sophia, and the best time to visit is simply whenever the line looks short, especially on a hot summer day—it’s always cooler underground."