Deep under the old city of Istanbul is a nearly 10,000 square meter cistern that was built in the 6th century by the Emperor Justinian to provide water to the palaces and homes in the area. With 4 meter thick walls and a 9 meter ceiling, it had the capacity to store 100,000 tons on water brought in by aqueducts from the hills outside the city. Forgotten by many for a long time, it was brought back into the public consciousness in the 15th century by a French explorer who was intrigued by stories of local people fishing from their basements. Restored again in the 19th and 20th centuries, it is now open for visitors, who can escape the summer heat and walk around on catwalks suspended over the water, and marvel at the many styles of stone columns and the fish that still call the Cistern home. A truly unique site, just a block away from the Hagia Sofia. Don´t miss it!