The two large cities of the eastern Mediterranean, Constantinople and Alexandria, are known for the large number of ancient and medieval cisterns that are still preserved there. Based on the results of recent research projects carried out in these two cities, this article aims to provide a better knowledge of underground resources and to show how these contribute to shedding light on the urban history of Late Antiquity and of the later periods. The uniqueness of the Alexandrian hyponomoi is described in the hydrogeological context of the city. Recent studies have revealed the importance of long-term changes in water quality and the responses provided by new, larger cisterns between v e and viii e. century. The numerous cisterns known from ancient descriptions of Alexandria as well as from current research are believed to date back to at least the xth century. For Constantinople, recent research has shown that 209 cisterns are known in the historic peninsula: their distribution is analyzed and the cistern of the monastery of Stoudios chosen as a new case study since it is structurally prior to the basilica dated 463. L he development of this cistern and others from the same period presents an innovative form that can be found throughout the city. Comparison with recent studies by Salamis in Cyprus and Resafa in Syria allows us to understand the construction processes in the context of our knowledge of the first Roman cisterns in North Africa. Geochemical studies of aqueduct conduits offer a new perspective on the quality of water in Thracian developments, studied in relation to the function of large open cisterns.