It has long been expected that hydrogen becomes metallic under sufficient pressure, and researchers have been edging closer to its production. Now, Ranga Dias and Isaac Silvera of Harvard University in the US report that they have finally observed the bizarre quantum state. Squeezed in a diamond-anvil cell to a pressure of 495 GPa, hydrogen becomes a shiny metal with a plasma frequency of 32.5±2.1 eV at a temperature of 5.5 K – a transition predicted more than 80 years ago by Wigner and Huntington. The metal may be metastable, remaining metallic when the pressure is removed, similarly to how diamond is formed at high pressures and retains its form under normal conditions. If metastable, it could have applications as a rocket fuel far more powerful than liquid hydrogen and oxygen, and may even be superconducting at room temperature.