Water has remarkable effects on the properties of mantle rocks, but, owing to the high temperatures in the mantle, uncertainties remain about how and how much water is transported into the deep Earth. Recent studies have shown that stishovite and post-stishovites as high-pressure phases of SiO₂ have the potential to carry weight percent levels of water into the Earth’s interior along the geotherm of the subducting oceanic crust. As slabs are subducted to the deepest mantle, dehydration of these dense hydrous silica phases has the potential to change the physicochemical properties of the mantle by reducing melting points, forming new high-pressure phases, and enhancing the oxygen fugacity heterogeneity of the lower mantle.