The possible existence of a highly conducting mineral layer just above the boundary between Earth's core and mantle has gotten earth scientists a little excited. Such a layer would, by electromagnetic coupling, have a small influence on the exchange of angular momentum between the planet's rotating fluid core and its solid mantle. Theorists say this tiny difference could explain why the length of an Earth day fluctuates by a millisecond or so year-to-year and could contribute ever so slightly to Earth's wobbling axis of rotation. Kei Hirose and Kenji Ohta at Tokyo Institute of Technology and coworkers present a chemical model that provides the first direct conductivity measurements of the proposed mineral (Science 2008, 320, 89). The team prepared samples of perovskite, a common magnesium-iron silicate mineral, and tested its electrical conductivity in a laser-heated diamond anvil cell. At the high temperature and pressure expected at the core-mantle boundary, the scientists observed the mineral undergo a phase transition to form "postperovskite." This transition is accompanied by a shift in the electronic structure of metal atoms, causing the conductivity to jump by two orders of magnitude. It is a postperovskite layer, perhaps 300-km thick, just above the core-mantle boundary that could give rise to Earth's observed rotational fluctuations, the researchers say.