Museum Cistern of Philoxenos in Fatih

You're standing at the Cistern of Philoxenos, in the Sultanahmet district of Istanbul. From the street you'd hardly notice it, but beneath your feet lies a huge Byzantine water reservoir. It sits between two landmarks of old Constantinople: the Forum of Constantine and the Hippodrome. After the famous Basilica Cistern, this is the second largest cistern in the city.