As the largest of several hundred ancient cisterns that lie beneath the Istanbul, Basilica Cistern was built in the 6th century. Water from the enlarged cistern was used for filtration in the Great Palace of Constantinople and other buildings on the First Hill, and it continued to serve the Topkapi Palace after the Ottoman conquest in 1453 and until the present day. This building is 140 meters long and 70 meters wide, with brick walls nearly 5 meters thick. Its brick floor is covered in Khorasan mortar to make it waterproof. It is also capable of holding 100,000 tons of water.

The cistern is located approximately 150 meters (490 ft) southwest of the Hagia Sophia on the historic Peninsula of Sarayburnu. It was called Basilica in Greek due to its location under a large public square on the First Hill of Constantinople, the Stoa Basilica.

The Basilica Cistern is an architectural marvel with 336 marble columns arranged in 12 rows, each standing 9 meters (30 feet) high. Many of the columns were repurposed from older Roman structures, including two notable columns with Medusa heads as their bases, believed to be placed upside down to ward off evil spirits. This stunning underground reservoir has become a popular tourist attraction, known for its eerie ambiance and historical significance, offering visitors a unique glimpse into Byzantine engineering.