The Earth’s interior is a hidden and significant water reservoir on a par with the hydrosphere. The properties and processes of the crust and the mantle are strongly modulated by the storage and transport of water. Despite significant progress made in the research of deep water cycle over the past three decades, important scientific challenges still remain. How much water is down there? How heterogeneously is water distributed on different spatial scales? Where did water originate? How has water been exchanged between external and internal reservoirs of the Earth through geological time? How significant, quantitatively, is the role of water in facilitating mantle convection and melting? What is the function of water in ore formation, deep earthquakes, and volcanic eruptions? We invite contributions from experimental, computational, analytical, petrological-geochemical and geophysical studies that progress on all aspects of water as a vital element of global geodynamics.