Metallic hydrogen is a sort of super-compressed hydrogen found in the cores of gas giants and stars. As hydrogen tops the Periodic Table's alkali metal column, it has been known for a while it has the potential to be a metal, but only under extreme pressures. Metallic hydrogen is crushed so closely that the atomic nuclei are separated only by a dense electron soup which flows between them. It is significantly less dense than neutronium, however, where the electrons merge together with the protons in hydrogen to make neutrons. Like all metals, this one is conductive and requires an electrical current to measure the presence of metallization.
This material was only synthesized under laboratory conditions as recently as 1996, at the Lawrence Livermore National Laboratory. It only existed for about a microsecond, and required temperatures of thousands of degrees and pressure over a million atmospheres to achieve. This was a surprise, as it was previously thought that solid (very cold) hydrogen was required to produce metallic hydrogen. Previous experiments subjected solid hydrogen to pressures up to 2.5 million atmospheres, with the absence of any detectable metallization, so the experiment involving the compression of hot hydrogen was set up to measure other properties of the material, not with the intention of producing metallic hydrogen. Nevertheless, that's how it was first made.
Although the metallic hydrogen produced at the Lawrence Livermore National Laboratory was solid, it has been theorized that it may be possible to create a liquid version, if even greater pressures, around 4 million atmospheres, are used. Calculations have also determined that this material might be a superconductor at room temperature, although this property would be somewhat useless for practical purposes, as the cost of compressing something to over a million atmospheres for an extended period of time is much greater than cooling something down to near absolute zero. However, there is a small chance that metastable metallic hydrogen might be possible -- that is, one that retains its phase even when the pressure is removed.
It is thought that metallic hydrogen exists in the cores of the larger gas giants in our solar system: Jupiter and Saturn, as well as a hydrogen shell near the core of the Sun.