The basilica Cistern is one of the must-see stops in Istanbul. Also called the Underground Palace, it is the most impressive water storage facility found in Istanbul. Extremely impressive is the walk underground among the 9-meter columns and the huge stone heads of the Gorgon Medusa. The life of a large part of former Constantinople depended on this reservoir. The name of the cistern is due to the fact that there was once a basilica on top. The facility itself was built during the reign of Emperor Justinian in the 6th century. Several hundred such tanks have been found in Istanbul, but this is the largest. According to historical sources, more than 7,000 slaves were used to build it. It was filled with water from a spring nearly 20 km away. Due to the city’s constant water supply, it was so difficult to conquer. At one end of the tank, two of the columns end with the heads of a Gorgon Medusa. The columns are believed to have been taken from a ruined temple from an earlier era. One Medusa is turned upside down – it is believed that this is done to reduce the power of the mythological creature’s gaze. However, it is possible that purely practical reasons lie behind this decision of the tank builders.