Several recent studies have reported hydrous minerals to be stable at pressure and temperature conditions representative of Earth’s deep interior, implying that surface water may be transported to the deep mantle. However,the dehydration decomposition of hydrous minerals generally occurs at pressures before reaching those corresponding to the deep lower mantle. In this study, stabilities of hydrous phases, FeOOH and AlOOH, have been studied at the pressure and temperature conditions of the deep lower mantle, based on first-principles calculations and in situ X-ray diffraction experiments. The formation of a new FeOOH and AlOOH phases, which are stable at the conditions of the base of the mantle, have been observed.