This is an unusual site, and although it doesn’t take long to tour, it is definitely worth a look.
Built during the reign of Emperor Justinian in the 6th century, it covers an area about the size of two football fields, and held 27 million gallons of water.
I read in the Rick Steves guidebook that until the wooden walkway was built a number of years ago, the water was 6 feet deep and visitors rented a boat and rowed around to tour the cistern. Maybe you remember the boat scene in the movie From Russia with Love?