One of Istanbul's most majestic historical monuments is undoubtedly the Basilica Cistern, built during the reign of Justinian I. It's sometimes referred to as the "Sunken Palace" due to the columns which rise up from the water. The "Basilica" part of the name comes from the fact that a basilica once stood on the spot where it was built.
The Basilica Cistern is 140 meters long by 70 meters wide and is reached by a 52-step staircase. Once inside, you are suddenly faced with 336 separate columns, each measuring 9 meters high, which are organized into 12 rows of 28 columns. Most of the columns were carved from a single piece of marble.The ground is made of bricks covered in Horsan plaster and can hold up to 100,000 tons of water. If you walk to the end and descend the steps, you'll find two columns whose bas-reliefs depict the head of Medusa. It's a curious find since no one knows exactly how or why it came to be.