The planet's most prolific atmospheric oxygen source has just given up its most detailed mug shot yet. Jim Barber, a professor at Imperial College London, and others have peered into the complex structure and chemistry of Photo-system II's water-splitting manganese and calcium core, the heart of photosynthesis. PSII uses light energy to derive electrons from water for reducing plastoquinone, a mobile electron carrier. Electron microscopy; and more recently X-ray crystallography, have yielded increasingly clearer details of the cluster of four manganese ions and either one or two calcium ions at PSII's center, but without revealing the exact geometry or sequence of events as water is split into protons, electrons, and oxygen. Barber's group has produced an X-ray crystallographic structure at 3.5 Å, the highest resolution yet obtained for PSII, published online this month.[1]