Geochemical studies of volcanic rocks from Baffin Island, Canada, and Iceland have uncovered hints that some of the water from Earth's earliest days may still be trapped deep inside the planet. This discovery comes from an international research team led by the University of Bayreuth, which has identified two new iron compounds—Fe₅O₁₂Hₓ and Fe₇O₁₂Hₓ—as potential candidates for storing water in the Earth's deep interior. These compounds are the first realistic examples of materials that could hold water under extreme conditions found far below Earth's surface. To simulate the intense pressure and heat of Earth's deep mantle, the researchers used a technique called diamond anvil cell compression. Tiny samples were squeezed between two diamonds and then heated with lasers to reach pressures of 78 to 198 gigapascals and temperatures of 2,400 to 2,800 kelvin—conditions similar to those found several thousand kilometers beneath the surface. Using synchrotron X-ray radiation, the team analyzed the chemical makeup and crystal structures of the compounds formed under these conditions. Remarkably, these iron-rich compounds formed not only from water-rich samples but also from nearly water-free mixtures, suggesting that even trace amounts of hydrogen can stabilize them. One potential source of ancient water is a primordial magma ocean that once existed above the boundary between Earth's core and mantle. As this ocean solidified, leftover melt may have become enriched in both water and iron—precisely the ingredients needed to form the newly discovered minerals. According to Professor Leonid Dubrovinsky of the Bavarian Research Institute of Experimental Geochemistry and Geophysics (BGI), these dense compounds would likely remain near the core-mantle boundary for long periods, trapping water there. The Fe₅O₁₂Hₓ compound, in particular, is highly hydrogen-rich. Researchers estimate that just 1% of this mineral in the mantle could hold a significant portion of Earth's total hydrogen inventory. The formation of these oxyhydroxide compounds could also affect the structure of the mantle. As they form, they extract iron from surrounding silicate minerals, potentially shifting boundaries between different mantle layers. This might explain unusual seismic signals detected near the core-mantle boundary. Additionally, these water-rich minerals may act as a "volatile fuse" for mantle plumes—upwellings of hot material from deep within Earth. If these compounds melt more easily than surrounding silicates during heating from the core, they could release stored water and other volatile substances into rising magma, linking deep reservoirs with volcanic activity at the surface.