Built in the 6th century by Byzantine emperor Justinian, the cathedral-sized underground water reservoir is one of Istanbul’s most popular and probably most eerie sights, inhabited by goldfishes dwelling in permanent half-light between ancient pillars.

Although several other cisterns that been excavated, the Yerebatan Sarnıcı (Sunken Cistern) is one of the largest and most extensively restored structure of its kind in Istanbul. Water was brought in from the Belgrade Forest in the north over miles long aqueducts, and the cistern could hold up to 80,000 cu of water. 336 columns, arranged in 12 symmetrical rows of 28 columns, support the cistern’s roof. As part of the restoration, wooden walkways were added which enable visitors to wander among the atmospheric lit aisles.

Inside the cistern, two columns receive special attention for their extraordinary base stones. Hidden away in the northwestern corner, they each rest on giant Medusa heads. While one head is turned upside down, the other is turned sideways. Legend has it this measure was taken to negate the Medusa’s deathly look.