The Basilica Cistern is an ancient underground water reservoir located in Istanbul, Turkey. Built in the 6th century during the reign of Emperor Justinian I, it was designed to store water for the nearby Great Palace of Constantinople. The cistern features a vast, vaulted ceiling supported by 336 columns, creating a serene and atmospheric environment. Visitors can explore the cistern's pathways and admire its unique architecture, including the famous Medusa heads used as column bases. The structure holds approximately 80,000 cubic meters of water and remains one of the most significant examples of Byzantine engineering and artistry.