<p>The Earth’s deep interior likely holds water equivalent to several oceans, forming a dynamic deep water cycle driven by plate subduction and volcanism. Because water’s physical and chemical properties are modulated by thermodynamical conditions, its distribution throughout the Earth is heterogeneous. The mantle transition zone, ranging from 410 to 660 km, is a water reservoir. Studies have shown that deeply subducted plates can penetrate the bottom of the mantle transition zone, allowing water to reach the lowermost mantle, approximately 2900 km deep. This work summarizes recent progress in studying water and hydrous mineral phases in the lower mantle (depth 660‒2900 km). It covers topics such as deep hydrous phases, water-mineral reactions, and their implications for the deep water cycle. We particularly focus on how the chemical properties of water molecules are altered under the high temperature and pressure conditions of the deep mantle, ultimately leading to a paradigm shift in our understanding of deep water-mineral chemistry. Specifically, hydrous minerals in the shallower depths are formulated upon hydroxyl group composed of O^‒2 and H¹. In stark contrast, oxygen in the deep lower mantle may exist in the chemical valence state of O^‒1, and hydrogen could become a highly mobile superionic ion. This means oxygen, conventionally a rock-forming element, gradually exhibit volatile characteristics. This novel chemical phenomenon indicates the formation of unexpected stoichiometry and phases in the deep mantle, which may induce abnormal geophysical properties such as low seismic velocity and high conductivity. These findings can help us trace the deep water cycle and may indirectly regulate Earth’s habitable surface environment.</p>