Subducting slabs transport water stored in hydrous minerals into the transition zone and lower mantle. The water storage capacity of the upper and lower mantles is less than 0.2 wt%. The transition zone has a storage capacity of approximately 0.5-1 wt% due to a water solubility of about 1-3 wt% in wadsleyite and ringwoodite, which are the major constituents of the transition zone. Thus, the transition zone may be a major water reservoir in the Earth's interior. Recent geophysical observations suggest the existence of the hydrated transition zone beneath subduction zones. Water or hydrogen may be transported as far as the bottom of the lower mantle by reacting with metallic iron in the lower mantle to form hydrous phases or iron hydride.