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The origin of water on the Earth is a long-standing mystery, requiring acomprehensive search for hydrous compounds, stable at conditions of the deepEarth and made of Earth-abundant elements. Previous studies usually focused onthe current range of pressure-temperature conditions in the Earth's mantle andignored a possible difference in the past, such as the stage of the core-mantleseparation. Here, using ab initio evolutionary structure prediction, we findthat only two magnesium hydrosilicate phases are stable at megabar pressures,$\alpha$-Mg$_2$SiO$_5$H$_2$ and $\beta$-Mg$_2$SiO$_5$H$_2$, stable at 262-338GPa and >338 GPa,respectively (all these pressures now lie within the Earth'siron core). Both are superionic conductors with quasi-one-dimensional protondiffusion at relevant conditions. In the first 30 million years of Earth'shistory, before the Earth's core was formed, these must have existed in theEarth, hosting much of Earth's water. As dense iron alloys segregated to formthe Earth's core, Mg$_2$SiO$_5$H$_2$ phases decomposed and released water.Thus, now-extinct Mg$_2$SiO$_5$H$_2$ phases have likely contributed in a majorway to the evolution of our planet.

Ultrahigh-Pressure Magnesium Hydrosilicates as Reservoirs of Water in Early Earth