When subjected to lower-mantle pressures and temperatures, natural “anhydrous” basalt containing 0.2 wt.% H₂O forms a phase assemblage in which SiO₂ stishovite is a significant carrier of hydrogen (up to 500 ppm H₂O by weight, as hydroxide), whereas the coexisting (Mg, Fe, Al, Ca)SiO₃ perovskite appears to be not (upper bound of 50 ppm (wt) H₂O). Contrary to the devolatilization characteristically observed at lower pressures, we find that the abundance of H₂O in residual stishovite increases from ∼100 to ∼400 ppm by weight upon partially melting the high-pressure mineral assemblage at 28–60 GPa. We infer that the trace concentration of Al within residual stishovite increases upon partial melting, thereby increasing the coupled abundance of H in this crystalline phase. The “anhydrous” component of subducted oceanic crust can thus recycle a significant amount of water into the lower mantle over the age of the Earth, with subducted stishovite potentially returning ∼10² times the amount of water present in today's atmosphere.