Under Pressure In order to understand the behavior of materials in the solid deep Earth, it is important to be able to estimate how a material melts at high pressure. To this end, Fiquet et al. (p. 1516 ) performed experiments using a laser-heated diamond anvil cell coupled to in situ synchrotron measurements of peridotite rock—a mixture of minerals thought to represent Earth's upper mantle—across a wide pressure range. The results suggest that liquid phases may exist at very high pressure values, such that seismically anomalous zones near the boundary between the core and the mantle may result from isolated pockets of melt. Along similar lines, the base of primitive Earth's mantle may have acquired its trace element signature from partial melting of certain mineral phases higher up in the mantle.