Volcanic island and seamount chains form from deep-seated plumes of hot material upwelling through the mantle. The most famous of these is the Hawaiian-Emperor seamount chain. However, a large volcanic structure associated with a plume head that should precede the chain has long been missing. Wei et al. finally identified the likely location of this structure in the mantle under eastern Russia. The structure was likely subducted 20 million to 30 million years ago, and the location helps constrain several geodynamic processes.Science, this issue p. 983The Hawaiian-Emperor seamount chain that includes the Hawaiian volcanoes was created by the Hawaiian mantle plume. Although the mantle plume hypothesis predicts an oceanic plateau produced by massive decompression melting during the initiation stage of the Hawaiian hot spot, the fate of this plateau is unclear. We discovered a megameter-scale portion of thickened oceanic crust in the uppermost lower mantle west of the Sea of Okhotsk by stacking seismic waveforms of SS precursors. We propose that this thick crust represents a major part of the oceanic plateau that was created by the Hawaiian plume head ~100 million years ago and subducted 20 million to 30 million years ago. Our discovery provides temporal and spatial clues of the early history of the Hawaiian plume for future plate reconstructions.