With its 170 x 147 meters this is the largest of the open-air cisterns. It was constructed under Emperor Anastasius (491-518) and fell into disuse in later Byzantine times. It was named after a famous church nearby, the St. Mocius (Mocius was a local martyr under Diocletion). Strolling through Istanbul mentions it was used as a vegetable garden and orchard, but when I visited it was a nice park and playground, a very hospitable place.