A cistern is a space, open or closed, used to collect rainwater or store clean water. Cisterns were used in the Middle East from the earliest times, thus making it possible to collect water and protect it from evaporation and impurities. Romans excelled in the architecture of water systems, and the Byzantine took the art even further. More than one hundred cisterns are known to have existed in Constantinople. The Basilica cistern, now known as the Yerebatan cistern, is İstanbul’s largest closed cistern and was built in the mid-500s. Its name of “Basilica” comes from the fact that it was originally beneath a Byzantine basilica. Its vault is supported by 336 columns, and its large size has earned it the popular name Yerebatan Sarayı (Sunken Palace). It was used as a cistern for some time after 1453, to provide water for the Topkapı Palace gardens, but Islamic tradition prefers running to stagnant water and it was thus eventually abandoned. The structure was forgotten for a time, until the French explorer Petrus Gyllius rediscovered it in 1544–1550. Noticing that some homes drew water from wells in their basements, he descended into a well and found the magnificent structure. The cistern has been a museum since 1988.