This image shows the predicted optical absorption of a 1 μm of hydrogen in a high pressure diamond anvil cell for different crystal structures at a pressure of 300 GPa (3 million times normal atmosphere—similar to the pressure in the center of the Earth). At these pressures hydrogen no longer forms molecules, but instead forms in sheets, as shown in the figure. Scientists use optical absorption to look for metallization in hydrogen, based on the assumption that metallic hydrogen would be opaque as most metals are. But the team's analysis shows that it may very well actually be transparent. Absorption units on the graph (AU) are in factors of 10, meaning 2 AU lets just 1% of the incident light pass through the structure (quite dark!). The graphite structure is an ideal structure that is not expected to be observed in reality. The proposed high-pressure forms, phase 3 (at low temperatures) and phase 4 (at room temperature), are both predicted to be transparent in the near infrared and optical frequencies of light, although phase 4 is poor metal. The Cmca structure is a similar structure, but is predicted to be a better metal and opaque, and to form at higher pressures. Graph is courtesy of Ronald Cohen.