LABORATORY results demonstrating that basic to ultrabasic melts become denser than olivine-rich mantle at pressures above 6 GPa (refs 1-3) have important implications for basalt petrogenesis, mantle differentiation and the storage of volatiles deep in the Earth. A density cross-over between melt and solid in the extensively molten Archaean mantle has been inferred from komatiitic volcanism4-6 and major-element mass balances7, but present-day evidence of dense melt below the seismic low-velocity zone is lacking. Here we present mantle shear-wave impedance profiles obtained from multiple-ScS reverberation mapping for corridors connecting western Pacific subduction zone earthquakes with digital seismograph stations in eastern China, imaging a ∼5.8% impedance decrease roughly 330 km beneath the Sea of Japan, Yellow Sea and easternmost Asia. We propose that this represents the upper surface of a layer of negatively buoyant melt lying on top of the olivine→β-phase transition (the 410-km seismic discontinuity). Volatile-rich fluids expelled from the partial melt zone as it freezes may migrate upwards, acting as metasomatic agents8,9 and perhaps as the deep 'proto-souree' of kimberlites10,11. The remaining, dense, crystalline fraction would then concentrate azone.