The core-mantle boundary is an area of complex thermal, seismic and chemical structure. Of particular interest are ultra-low velocity zones, areas of very low seismic velocities on the core-mantle boundary that may suggest melt is present. The identity of this melt, and its solid complement, is still an open question. Deng et al.  suggest that the phase may be molten hydrogen-bearing iron peroxide (FeO2Hx), a pyrite-like structured, hydrated mineral. If true, ultra-low velocity zones would represent a significant reservoir for hydrogen in the mantle.
Citation: Deng, J., Karki, B. B., Ghosh, D. B., & Lee, K. K. M. . First‐principles study of FeO2Hx solid and melt system at high pressures: Implications for ultralow‐velocity zones. Journal of Geophysical Research: Solid Earth, 124, 4566– 4575. https://doi.org/10.1029/2019JB017376
—Stephen W. Parman, Editor, JGR: Solid Earth