The underground palace is an underground building next to the Hagia Sophia. It was originally used for water storage in the Imperial Palace and has a capacity of up to 100,000 tons. It was also used as a military ammunition depot. In fact, it is not big here. You can go back and forth in half an hour. It is not in the museum package. You need to purchase tickets separately. Many people will ignore this attraction. However, there is an unsolved mystery inside: there are two mysterious Medusa heads under the water column. Why is one side placed on the side?