Italy Piscina Mirabilis

Piscina Mirabilis is the largest and most monumental cistern of drinkable water ever built by the ancient Romans. It had the function of supplying water to the numerous ships belonging to the Classis Misenensis of the Roman Navy, once moored in the nearby port of Miseno. The cistern, also known as "the Water Cathedral " for its largeness and majesty, is located in Bauli, the name that the ancient Romans gave to the current city of Bacoli. (from https://piscinamirabilisbacoli.it/en/piscina-mirabilis)

By train: From Montesanto station in Naples, take the EAV train “Cumana” Line L9 to the terminus of Torregaveta. Then, take the "Baia - Torregaveta" bus and get off at the "Bacoli - Centro" stop. From here, follow the pedestrian path/Walkway. Journey time is about 40 minutes.