The Portuguese Cistern is an underground water storage system located in the city of Istanbul, Turkey. Built in the 16th century during the reign of the Ottoman Empire, it was designed to supply water to the nearby Topkapi Palace. The structure features a series of columns and arches, creating a vast, cavernous space that can hold a significant amount of water. This cistern is notable for its unique architectural style, which reflects the influence of Portuguese design. Visitors can explore the dimly lit chambers, where the atmosphere is enhanced by the presence of fish swimming in the water. The cistern remains an important historical site, showcasing the engineering skills of its time.