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Since 1967 two major advances in our knowledge of the core and core-mantle boundary have occurred. First, the giant large-amplitude seismic array (Lasa) in Montana provided convincing observations of reflections (PKiKP) from the inner-core boundary (ICB); second, evidence grew that a low-velocity shell existed in P and S above the mantle-core boundary (MCB).

After the recent energetic studies of the upper mantle, there are signs of quickening geophysical interest in the physical state of the earth's core. The precise nature of the striking structural discontinuities that define the MCB and ICB is of importance to solid-state physics, to geochemistry, and to geophysics. In particular, laboratory shock-wave results must be matched against the seismological inferences.