A new study is challenging one of the most persistent assumptions about Earth’s deep interior: that the planet’s lower mantle may contain vast amounts of water locked inside its dominant mineral. Geophysical evidence presented by Y. Okuda, K. Ohta, C.E. Mohn and colleagues indicates that bridgmanite, the most abundant mineral in Earth, may be nearly dry throughout much of the lower mantle. The finding could force scientists to rethink how water is stored, transported and cycled inside the planet, while reshaping ideas about the deep mantle’s chemistry, dynamics and evolution.
Bridgmanite is a high-pressure form of magnesium silicate, with a chemical composition broadly related to MgSiO₃. It is stable only under the extreme pressures and temperatures found deep beneath Earth’s surface, primarily between roughly 660 and 2,900 kilometers below ground. Because it makes up an estimated majority of the lower mantle, even small amounts of water incorporated into its crystal structure would have enormous global significance. The mineral has therefore been at the center of a long-running debate over whether the lower mantle acts as a hidden reservoir capable of holding several oceans’ worth of water.
Water in the mantle does not necessarily exist as liquid or as familiar ice. At extreme pressures, hydrogen can enter the atomic structure of minerals, replacing or occupying positions within their crystal lattices. These hydrogen-bearing defects can alter a mineral’s density, electrical conductivity, elasticity and ability to deform. Scientists can use those properties to search indirectly for water at depths that cannot be reached by drilling. The new study focuses on this geophysical fingerprint, asking whether observations from inside Earth are consistent with a strongly hydrated bridgmanite or with a mineral that contains very little hydrogen.
The distinction matters because water changes the behavior of the mantle even when it is present in microscopic quantities. Hydrogen can weaken mineral grains, affect how heat moves through rock and influence the viscosity that controls mantle convection. Convection is the slow circulation of solid mantle material over geological timescales, driven by heat escaping from Earth’s interior. It helps power plate tectonics at the surface and transports chemical material between shallow and deep parts of the planet. If bridgmanite is nearly dry, the lower mantle may be stronger, less chemically reactive and less efficient at storing water than many models have assumed.
The researchers’ conclusion is based on geophysical evidence rather than direct samples from the lower mantle. Rocks from those depths are rarely brought to the surface, and material recovered from volcanic eruptions generally represents shallower regions or may have been altered during ascent. Instead, scientists compare the predicted physical properties of minerals under lower-mantle conditions with signals recorded at Earth’s surface. Seismic waves, for example, travel through the planet at speeds controlled by the density, elasticity and temperature of the materials they cross. Small changes in composition or crystal structure can leave measurable effects in those signals.
Electrical conductivity offers another possible window into the deep Earth. Hydrogen-bearing minerals can conduct electricity differently from their dry counterparts, allowing electromagnetic observations and laboratory measurements to place limits on the amount of water hidden in the mantle. Density and seismic velocity provide additional constraints. A model that contains substantial hydrogen must still reproduce the observed behavior of the lower mantle, including how seismic waves propagate and how the interior responds to changes in pressure and temperature. The evidence highlighted by Okuda and colleagues points toward a scenario in which bridgmanite contributes little water compared with some earlier estimates.
The result does not mean that the lower mantle is completely free of water, nor does it eliminate the possibility that other minerals or chemically distinct regions contain significant hydrogen. The mantle is not uniform. It includes temperature variations, compositional boundaries, recycled slabs of oceanic crust and material that may have remained isolated since Earth’s formation. Other minerals, especially those concentrated in particular chemical environments, could carry water more effectively than bridgmanite. The study’s central message is more precise: the planet’s dominant lower-mantle mineral may not be the enormous water reservoir that its abundance once seemed to imply.
That possibility has major consequences for the global water cycle. At Earth’s surface, water is exchanged among the oceans, atmosphere, crust and living systems. Plate tectonics can carry hydrated minerals downward at subduction zones, while volcanic activity returns some water to the surface. If bridgmanite is unable to retain much hydrogen under lower-mantle conditions, water transported deep into Earth may be released at shallower depths, trapped in other phases or redistributed through chemical reactions. The deep water cycle could therefore be more localized and episodic than a simple model in which the lower mantle steadily absorbs water.
The finding may also help explain why the lower mantle behaves differently from the upper mantle. The two regions are separated by a major pressure-driven mineral transition near 660 kilometers depth. Minerals that are stable above this boundary can transform into denser structures below it, changing how elements and volatiles move through Earth. A nearly dry bridgmanite phase would influence the strength of this barrier, the circulation of subducted material and the rate at which heat is transferred from the core toward the surface. These effects could extend from microscopic defects in crystals to the evolution of the entire planet.
The study arrives at a moment when scientists are increasingly using Earth’s interior as a natural laboratory for extreme materials physics. Experiments in diamond-anvil cells, high-temperature presses and advanced computational simulations can recreate pressures equivalent to those found hundreds or thousands of kilometers underground. When those results are combined with seismic and electromagnetic observations, researchers can test competing models of the mantle without ever accessing it directly. The evidence for nearly dry bridgmanite will likely stimulate new experiments and sharper searches for the hydrogen that must still exist somewhere within Earth’s interior.
The implications reach beyond mineralogy. Water affects melting, volcanism, the mechanical strength of rocks and the long-term habitability of a planet. Understanding where Earth’s water resides is essential for reconstructing how the planet formed and how its surface environment became stable enough for life. If the lower mantle has less capacity to store water than previously believed, the balance between surface reservoirs and the deep interior may have been different throughout geological history. The new work does not close the debate, but it delivers a striking message: one of Earth’s most abundant minerals may be far less watery than expected, and the planet’s hidden water may be distributed in more complicated ways than its deepest rocks suggest.
Subject of Research: The water content and geophysical properties of bridgmanite in Earth’s lower mantle.
Article Title: Geophysical evidence of a nearly dry bridgmanite in the Earth’s lower mantle.
Article References: Okuda, Y., Ohta, K., Mohn, C.E. et al. Geophysical evidence of a nearly dry bridgmanite in the Earth’s lower mantle. Nature Communications (2026). https://doi.org/10.1038/s41467-026-76621-1
Image Credits: AI Generated
DOI: 10.1038/s41467-026-76621-1
Keywords: bridgmanite, lower mantle, Earth’s interior, deep water cycle, mineral physics, geophysics, mantle dynamics, hydrogen, seismic evidence, planetary science
