The Cistern of Mocius was one of the several hundred cisterns underneath the city of Constantinople.
Built on the Seventh Hill by the Roman Emperor Anastasius in the 6th century, the Cistern of Mocius was fairly large by Constantinople's standards, and notable for the fact that it is one of the few cisterns above ground that are still visible today.
When Byzantium, later renamed Constantinople, became the capital of the Roman Empire, it soon had more inhabitants than it could supply with the water of its wells and the little river west of it, leading to large cisterns being constructed. One of these, built by the emperor Anastasius I on the Seventh Hill, was the "Cistern of Saint Mocius", named after a saint who was venerated in a nearby church.
Currently, the Cistern of Mocius is the youngest of the great cisterns.