Water is not merely a surface phenomenon. Although oceans, ice caps, and groundwater form the visible part of the hydrological cycle, a hidden reservoir of water is stored within the rocks of the mantle, bonded into the crystal structures of nominally anhydrous minerals and discrete hydrous phases. Geophysicists have long recognized that the Earth’s interior exchanges water with its surface over geological time, a process that regulates volcanism, seismicity, and the evolution of the oceans. Estimates of how much water resides in the deep mantle, however, span an enormous range, precisely because researchers cannot agree on which minerals can retain hydrogen under lower-mantle conditions of extreme pressure and temperature.

The commentary frames the problem against the backdrop of the Earth’s early history. Work on ancient zircons, melt inclusions, and primitive basalts suggests that the planet acquired much of its volatile inventory very early, possibly from primordial sources rather than exclusively from late-arriving comets or asteroids. If a substantial fraction of the Earth’s water was incorporated into the mantle during accretion, then the deep water cycle must be capable of retaining and redistributing that water over billions of years. Conversely, if the mantle is largely dry, then subduction must efficiently return most of the water it carries back to the surface. Both scenarios carry profound implications for mantle convection, the oxidation state of the deep Earth, and the long-term stability of the oceans.

The traditional carriers of water in the mantle are well catalogued down to the transition zone. Serpentine, a hydrous magnesium silicate formed when seawater alters oceanic crust and lithosphere, decomposes at shallow depths. Its breakdown products, dense hydrous magnesium silicates such as phase D, were once thought to shepherd water downward. Laboratory experiments showed that these phases can persist to the top of the lower mantle, roughly 660 kilometers deep, and even deeper in colder subducting slabs. Yet as temperature and pressure rise through the lower mantle, most of these hydrous silicates destabilize, releasing water into melts or fluids. Whether that water can be fixed into a stable crystalline host deeper down has been a puzzle for decades.

A key development came with the discovery that water itself, in the form of hydroxyl, can enter the crystal lattice of the lower mantle’s dominant minerals. Experiments by Li and colleagues reported in Nature Geoscience in 2020 demonstrated that bridgmanite, the most abundant mineral in the Earth, can incorporate substantial hydroxyl under lower-mantle conditions, while studies of the high-pressure ice phase Ice VII, synthesized and observed in inclusions in diamonds, showed that free water can exist as a crystalline solid at transition-zone depths. Zhang and coworkers had earlier identified a hydrous layer in silicon-deficient bridgmanite, while Nishi and colleagues reported a pyrope-type hydrous phase stable near the core-mantle boundary. Each discovery suggested multiple potential reservoirs, but none settled the question of what actually transports water across the 660-kilometer boundary in the first place.

The new experiments reviewed in the commentary focus on iron oxyhydroxides, phases closely related to the familiar surface minerals goethite and hematite but squeezed and heated to lower-mantle conditions. Under extreme compression, FeOOH phases adopt crystal structures stabilized by hydrogen bonds and can retain hydrogen at pressures corresponding to depths greater than 1,500 kilometers. Crucially, when combined with aluminum or other elements, these phases appear to remain stable at the elevated temperatures of the deep mantle, something the pure iron endmember cannot achieve. In slabs cold enough to suppress thermal breakdown, iron oxyhydroxides could therefore act as water ferries, carrying hydrogen from the seafloor to the base of the mantle.

Experiments led by researchers including Ohtani and Ishii, and summarized in recent work in Progress in Earth and Planetary Science, show that FeOOH synthesized in the laser-heated diamond anvil cell retains its hydroxyl content even when subjected to lower-mantle temperatures, particularly when the starting materials reflect the compositions of subducted banded iron formations—iron-rich sediments deposited on ancient seafloors. Ishii and colleagues also identified post-stishovite and other dense silica phases that can host aluminum and hydrogen, extending the suite of potential deep-water carriers. Together, these results sketch a pathway in which chemically heterogeneous, cold subducted material delivers water into the lower mantle where earlier models predicted it should escape.

Independent lines of evidence lend the experimental picture geological credibility. Ultra-deep diamonds, formed at depths exceeding 600 kilometers, frequently trap inclusions of Ice VII and hydrous ringwoodite, direct snapshots of free and bound water at transition-zone depths. Seismological observations of discontinuities, attenuation anomalies, and low-velocity features near the core-mantle boundary have been interpreted as signs of partial melting triggered by hydrous phases releasing water at the base of the mantle. Studies by Karato and colleagues have clarified how even small amounts of hydrogen alter the viscosity, electrical conductivity, and seismic anisotropy of mantle minerals, providing geophysical tools to test where water accumulates. The consistency between laboratory phase diagrams and geophysical signatures strengthens the case that the deep water cycle is active and dynamic rather than a geological curiosity.

Still, the commentary emphasizes that the puzzle is far from solved. The stability fields of iron oxyhydroxides depend sensitively on temperature, oxygen fugacity, and composition, and the conditions inside subducting slabs vary enormously. Estimates of the hydrogen-storage capacity of bridgmanite and ferropericlase continue to be revised, and the partitioning of water between minerals, melts, and fluids at lower-mantle pressures remains poorly constrained. Moreover, the total mass of subducted iron-rich sediments available to host FeOOH may limit how much water the oxyhydroxide pathway can deliver. Resolving these uncertainties will require new experiments at even higher pressures and temperatures, advanced synchrotron and spectroscopic techniques capable of detecting hydrogen in situ, and closer integration with seismic imaging of slabs penetrating the lower mantle.

What is at stake is nothing less than the budget of the Earth’s water. If iron oxyhydroxides and related phases can pump water into the deep mantle, the planet’s interior may hold an ocean’s worth of water or more, slowly exchanging it with the surface through volcanism and degassing. That reservoir would influence the oxidation state of the mantle, the generation of deep melts, and possibly the long-term evolution of the atmosphere. It would also mean that the familiar water cycle of rain, rivers, and evaporation is only the shallow skin of a vastly larger circulation that reaches within a few hundred kilometers of the Earth’s core. The new experimental results, as highlighted by Wilson, bring scientists closer to tracing the full arc of that hidden cycle, and remind us that one of the most familiar substances on Earth still keeps its deepest secrets hidden in plain sight, locked within minerals no one has ever seen with the naked eye.

Subject of Research: Transport and storage of water in the Earth's lower mantle by hydrous minerals such as iron oxyhydroxides

Article Title: The Earth’s puzzling deep water cycle

Article References: Wilson, A. J. (2026). The Earth’s puzzling deep water cycle. Nature Geoscience. https://doi.org/10.1038/s41561-026-02088-w

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02088-w

Keywords: deep water cycle, iron oxyhydroxide, lower mantle, subduction, hydrous minerals, bridgmanite, FeOOH, high-pressure experiments, mantle water reservoir, geodynamics, mineralogy, dense hydrous phases