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In the city of Istanbul, the ancient Constantinople, there are dozens of cisterns, ancient underground water reserves, under the old city and some have been restored and now they are open to the public. such as the Yerebatan Sarnıcı, the largest built in 532 AD by the Roman emperor Justinian I and fed by the Valens aqueduct, one of the longest aqueducts of the Roman Empire, formed by a cavity of 140 x 70 meters with 336 columns that support the ceiling 9 meters high each, and the Serefiye cistern, smaller than the previous one with only 32 columns but equally splendid. Today, thanks to a system of walkways suspended over the water, the restoration and setting works with a play of lights, visitors can enjoy a spectacle unique in the world. A visit to the cisterns is a must-see during a trip or holiday in Istanbul.
Useful links for more informations:
Istanbul Toursm Office: https://istanbul.ktb.gov.tr/EN-284674/istanbul.html
or
https://istanbul.com/travel/fountains-cisterns (English)
or
https://istanbul.com/it/travel/fountains-cisterns (Italiano)