As oxygen is squeezed and solidified under increasing pressure, it passes through a series of six distinct crystal phases, one of which−the ε phase−is marked by a dark red color and the collapse of magnetic behavior. The detailed structure of this phase, which has eluded researchers for 27 years, has now been determined independently by two groups using X-ray diffraction. Their results indicate that in the ε phase, which persists at pressures from 8 to 96 gigapascals, four O2 molecules associate into a rhombohedral O8 unit that is probably held together by weak chemical bonds. These (O2)4 rhombs (shown) are quite different from the long-sought O8 rings that would be an analog of the well-known S8 rings of elemental sulfur. The fact that this rhombohedral tetramer structure hasn't been predicted by theory "presents a challenge to our understanding of dense oxygen," according to a team of physicists from Scotland, Canada, and France, who report the structure in Nature (2006, 443, 201). The other structure elucidation of ε-oxygen was published last month by a Japanese team (Phys. Rev. Lett. 2006, 97, 085503).