Metallic Hydrogen can be found naturally at the heart of gas giants where the tremendous pressure crushes the atoms so closely that the nuclei are separated only by a dense electron soup. Hydrogen (being in the alkali metal column) has always been known to be a metal, but only under extreme circumstances. The process is similar to how neutronium is formed at the core of neutron stars, but Metal Hydrogen is nowhere near as dense. It's density is actually closer to that of water or iron (or somewhere in between).
Metal Hydrogen is a superconductor at room-temperature and is also usually a super-fluid, which gives the material special properties. The material is almost impossible to maintain in a metal state outside of gas giants though, unless it has been properly processed.
From Orion Wikia.com
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