We decided to visit the Basilica Cistern (Yerebatan Sarnıcı), one of two open to the public out of hundreds of cisterns in Istanbul. Most large, important Byzantine buildings provided for storage of water beneath them. The entrance to the Basilica Cistern of Istanbul is across the street from the Haghia Sophia, opposite the yellow building of the Tourist Police in Sultanahmet. This immense underground water container was built during the reign of Emperor Justinian I in 532 to meet the water needs of the Great Palace. The name of this subterranean structure derives from a large public square on the First Hill of Constantinople, the Stoa Basilica, beneath which it was originally constructed. Before being converted to a cistern, a great Basilica stood in its place, built between the 3rd and 4th centuries during the Early Roman Age as a commercial, legal, and artistic center. The basilica was reconstructed by Illus after a fire in 476. After cleaning and restoring the Basilica Cistern, the Istanbul Metropolitan Municipality opened it to the public in 1987. After descending into the underground water facility via a flight of stairs, you can take a stroll on the concrete walkways, enjoying the subdued lighting, the mysterious and beautiful forest of columns in the semi-darkness and the cool temperatures.