The Basilica Cistern, also known as the Yerebatan Sarnıcı in Turkish, is an ancient underground water storage system in Istanbul, Turkey. It was constructed in the 6th century during the reign of Emperor Justinian I of the Byzantine Empire to provide water to the city's Great Palace and other buildings in the area.

The cistern is over 140 meters long and 70 meters wide, and has a capacity of over 80,000 cubic meters of water. It is supported by 336 marble columns, each 9 meters tall, arranged in 12 rows. Many of these columns have been recycled from earlier Roman buildings.

The cistern was used continuously during the Byzantine period, and later by the Ottoman Empire. It was forgotten in the centuries that followed, and rediscovered in the mid-16th century by a scholar named Petrus Gyllius, who wrote about it in his book "Observations on the Subterranean Cistern of Constantinople."

Today, the Basilica Cistern is a popular tourist attraction, and visitors can walk through the dimly lit cistern and view its columns and the reflections of their images in the water. It has also been used as a location for films, music videos, and art exhibitions.