Under extreme pressure, sodium metal becomes optically transparent and transforms into a dielectric insulator. The transformation goes against normal physical expectations, according to the international team of physicists who report the discovery (Nature, DOI: 10.1038/nature07786). Deciphering the fundamental properties of highly compressed matter is important for understanding stars and giant planets. Yanming Ma of Jilin University, in China, and collaborators used diamond-anvil cells to condense sodium samples approximately fivefold, which forced the deep-core electrons in the samples' atoms to overlap, collapsing the metallic structure. At atmospheric pressure, sodium is silver in color, opaque, and exhibits a body-centered cubic packing structure. The team observed that sodium samples exposed to 2 million atmospheres (~200 GPa) of pressure first turned black and then became transparent. Combining spectroscopic and X-ray diffraction data collected from the dense samples with computational data, the researchers determined that the new transparent phase has a six-coordinate, highly distorted double-hexagonal close-packed structure. They attribute the insulating properties "to p-d hybridizations of valence electrons and their repulsion by core electrons into the lattice interstices." The researchers also suggest that other elements or compounds may exhibit the same insulating phase when their core electrons are forced to overlap.