What happens to water thousands of kilometres beneath our feet? An international research team has discovered two previously unknown iron compounds that remain stable even under the extreme conditions found deep within the Earth’s mantle. These iron oxyhydroxides could serve as long-term reservoirs for water near the boundary between the Earth’s core and mantle. The researchers examined the tiny crystals using intense X-rays from DESY’s light source PETRA III.

Water plays an important role deep inside the Earth: it influences for example how rocks melt and deform, as well as the transport of chemical elements. Until now, however, it was unclear which minerals can store water over long periods under the extreme conditions of the deep mantle. Geochemical studies of volcanic rocks from Baffin Island in Canada and from Iceland suggest that some of the water present since the Earth formed may still be stored deep inside the planet today.

To reproduce the extreme pressure and temperature conditions deep within the Earth’s interior in the laboratory, the researchers compressed tiny samples between two diamonds and heated them with lasers. In this experiment, they were able to reach pressures of almost 200 gigapascals – around two million times atmospheric pressure at the Earth’s surface – and temperatures of around 2,500 degrees Celsius. Under these extreme conditions, two previously unknown iron oxyhydroxides formed: compounds made up of iron, oxygen and hydrogen. Their chemical formulas are Fe₅O₁₂Hₓ and Fe₇O₁₂Hₓ.

“We cannot simply drill deep into the Earth and take samples, the deepest borehole reached only a depth of about 12 km in the earth crust” says Konstantin Glazyrin, Operation Manager at the high-pressure beamline P02.2 at DESY. Instead, researchers have to reproduce the conditions found inside the Earth in a depth of about 2,900 km, in the laboratory and then investigate which materials form at such extreme environment.

X-ray view of tiny crystals

To determine the chemical composition and crystal structures of the newly formed compounds, the research team used X-rays from PETRA III, among other techniques. The researchers had applied for beamtime at the high-pressure beamline P02.2 through the user proposal process.

Remarkably, the new compounds did not form only from water-rich starting materials. The researchers were also able to produce them from samples containing almost no water.

“It appears that even very small amounts of hydrogen are sufficient for these water-rich iron compounds to form,” concludes Leonid Dubrovinsky of the Bavarian Research Institute of Experimental Geochemistry and Geophysics (BGI) at the University of Bayreuth.

A potential water reservoir deep inside the Earth

The newly discovered iron oxyhydroxides could help store hydrogen under the extreme conditions of the deep Earth. In particular, Fe₅O₁₂Hₓ can accommodate large amounts of hydrogen.

Whether these compounds actually occur naturally in the Earth's interior remains an open question. The new findings nevertheless add another piece to the puzzle of understanding materials and processes under the extreme conditions of the deep Earth.

Further research will now explore how these compounds form and behave under such conditions. For DESY, the study also highlights how high-pressure experiments and synchrotron X-ray measurements can provide insights into materials that cannot be studied directly inside the Earth.

Researchers from the Centre for High Pressure Science and Technology Advanced Research (HPSTAR), the Bavarian Research Institute of Experimental Geochemistry and Geophysics (BGI) at the University of Bayreuth, the European Synchrotron Radiation Facility (ESRF), ETH Zurich, the Shanghai Institute of Applied Physics, DESY and Goethe University Frankfurt were among those involved in the study.

Original publication:

H. Yuan, L. Man, I. Kupenko et al. Dense iron oxyhydroxides as possible reservoirs of water under deep mantle conditions. Nature Geoscience (2026).
DOI: 10.1038/s41561-026-02067-1