Water, incorporated into minerals and melts at the high pressure and temperature conditions found in Earth’s deep mantle may constitute the planet’s largest geochemical reservoir of H2O, especially in the mantle transition zone at 410-660 km depth. At the atomic scale, hydration modifies the structure and physical properties of minerals through associated defects. At mesoscopic scales water influences diffusion, rheology and lattice preferred orientation. At geophysical scales, water cycling through the solid mantle plays a critical role in melt generation, plate tectonics, and may have acted to buffer the volume of Earth’s oceans over geologic time. I will focus on recent laboratory experiments, inclusions in diamond, and seismological observations that reveal clues about the distribution and origin of water in our habitable planet.