A recent study published in the journal Nature Geoscience has identified two new mineral phases—hexagonal iron oxyhydroxides with the formulas Fe₅O₁₂Hₓ and Fe₇O₁₂Hₓ—that might be stable under the extreme pressures and temperatures found at the base of the Earth's mantle. These minerals are unique because they can incorporate hydrogen into their crystal structures. This discovery could help explain the presence of ultra-low velocity zones (ULVZs), regions where seismic waves travel much slower than expected, located at the boundary between the Earth's mantle and core. The study does not confirm that these minerals exist naturally in the Earth's deep interior, but researchers suggest they could act as ancient reservoirs of hydrogen. These reservoirs might preserve chemical signatures from the early formation of the planet and from materials brought into the mantle through geological processes like subduction, where oceanic crust is pushed beneath continental plates. Due to their high density, these minerals might accumulate near the core-mantle boundary, potentially influencing the movement of volatile elements—such as water and carbon—between the mantle and the core. The composition of the Earth's mantle, which makes up more than 80% of the planet's volume, is still largely a mystery, especially in its deepest regions. While the transition zone in the mantle, between 410 and 660 kilometers deep, is believed to hold significant amounts of water, the upper and lower mantle have more limited storage capacities. Bridgmanite, the most common mineral in the lower mantle, has a very limited ability to hold water. However, the study does not confirm that the newly synthesized minerals are present in these regions. The researchers created these mineral phases in the laboratory by simulating the extreme conditions found in the Earth's deep interior. While this synthesis is a significant step, it remains to be proven whether these minerals form naturally in the Earth's mantle. The study adds to the ongoing scientific debate about the origin of ULVZs and the role of hydrogen and other volatile elements in shaping the Earth's internal structure.