Water is fundamental to the Earth’s surface environment. Volcanic outgassing and subduction recycling connect the deep interior with the atmosphere and oceans, shaping the planet’s long-term habitability. Surface oceans and mantle water are not separate reservoirs; they are inherently linked to the planet’s thermal and redox evolution.

In this talk, I will trace the storage and distribution of water in the mantle. Our earlier work showed that the capacity of the mantle to hold water depends strongly on temperature. A hotter Archean mantle could have held less water than the mantle actually contains today, suggesting that Earth's early surface might have been covered by larger oceans. The interplay between mantle cooling, water cycling, and sea-level stability highlights the importance of the mantle as a dynamic water reservoir. More recently, we examined how the disproportionation of iron in silicate minerals further regulates mantle water distribution. As the mantle becomes increasingly reduced with depth, metallic iron stabilizes and reacts with structural water in nominally anhydrous minerals, producing reduced hydrogen species such as molten FeHx or fluid H₂. This water–iron reaction prevents hydration of mantle silicates and drives hydrogen into metal-rich reservoirs, thereby altering the form and location of water in the mantle.

Together, this research provides a framework for understanding how much water the mantle can store and where it resides. The results suggest that the coevolution of water with Earth’s thermal and redox states governs its deep storage, distribution, and cycling, and, by extension, ocean volume, atmospheric composition, and the long-term redox evolution of the planet.