biostudies-literatureLi ZEuropean Research Council2787https://www.ebi.ac.uk/biostudies/studies/S-EPMC9120171biostudies-literatureUnknown13(1)The lowermost mantle right above the core-mantle boundary is highly heterogeneous containing multiple poorly understood seismic features. The smallest but most extreme heterogeneities yet observed are 'Ultra-Low Velocity Zones' (ULVZ). We exploit seismic shear waves that diffract along the core-mantle boundary to provide new insight into these enigmatic structures. We measure a rare core-diffracted signal refracted by a ULVZ at the base of the Hawaiian mantle plume at unprecedentedly high frequencies. This signal shows remarkably longer time delays at higher compared to lower frequencies, indicating a pronounced internal variability inside the ULVZ. Utilizing the latest computational advances in 3D waveform modeling, here we show that we are able to model this high-frequency signal and conNature communicationsKilometer-scale structure on the core-mantle boundary near Hawaii.PMC9120171No.804071-ZoomDeep804071Li ZCottaar SLeng KJenkins JfalseKilometer-scale structure on the core-mantle boundary near Hawaii.The lowermost mantle right above the core-mantle boundary is highly heterogeneous containing multiple poorly understood seismic features. The smallest but most extreme heterogeneities yet observed are 'Ultra-Low Velocity Zones' (ULVZ). We exploit seismic shear waves that diffract along the core-mantle boundary to provide new insight into these enigmatic structures. We measure a rare core-diffracted signal refracted by a ULVZ at the base of the Hawaiian mantle plume at unprecedentedly high frequencies. This signal shows remarkably longer time delays at higher compared to lower frequencies, indicating a pronounced internal variability inside the ULVZ. Utilizing the latest computational advances in 3D waveform modeling, here we show that we are able to model this high-frequency signal and con2022-01-01T00:00:00Z2022 May2025-04-22T00:29:52.859Z2025-04-05T19:34:09.077ZS-EPMC91201713558976510.1038/s41467-022-30502-5