Abstract: I will discuss the water cycling system in a plate-mantle dynamics simulation and give an implication for the lifetime of the ocean. An important ingredient for describing deep mantle water cycling is the solubility limit of the water in hydrous mantle minerals. By using the solubility limit in the thermodynamic estimate, the silicate mantle could hold a huge amount of water transported from the surface, ranging from 9 to 12 times as much as the present-day ocean mass. I will also discuss the controversial issue of how much the mantle could hold the water and its applicability to the surface environment of exo-planets.