The phase relations in the peridotite-water system to the top of the lower mantle reveal that a certain amount of water can be transported and stored in the transition zone by various hydrous minerals resulting in formation of a hydrous transition zone. Results of preliminary experiments to clarify the effect of water on kinetics of the olivine-wadsleyite transformation are given, and show that water enhances the transformation kinetics significantly. There are potential dehydration sites in the deep mantle; four sites along the descending slabs depending on their temperature profiles, i.e., the mantle wedge of the subduction zone, mantle transition zone, top of the lower mantle where hydrous ringwoodite and superhydrous phase B decompose, and deep in the lower mantle around 1200-1500 km depth where decomposition of phase D(G) occurs. We also expect one site of dehydration melting at the base of the upper mantle where the hot mantle plume hydrated in the transition zone can be dehydrated. The transition zone can serve as a water absorber for the mantle scale water circulation system. Some of the recent seismic works including the P-wave tomography images reveal the existence of low velocity anomalies in the transition zone suggesting possibly a wet transition zone. Elevation of the 410 km discontinuity and lack of clear evidence for a metastable olivine wedge in cold slabs may be consistent with the equilibrium phase transformation from olivine to wadsleyite in cold and wet slabs on the basis of our kinetic study of the transformation under wet conditions.