Metastable Metallic Hydrogen
When ordinary mono or diatomic hydrogen is compressed at pressures in excess of fifteen to twenty thousand atmospheres, internal repulsion between hydrogen atoms is overcome and metallic bonds are formed. This condition is "metastable", meaning that this substance only exists within a narrow range of circumstances, and will quickly revert to a more normal equilibrium with the right stimulus, such as compression in a fusion reactor. Nevertheless MMH is a very lightweight material, comparable in strength (though not density) to aluminum, and can be used as a structural material in some commercial applications.
MMH naturally occurs in stars and gas giants where the weight of the atmosphere and density of the gas become sufficient for its formation. At a depth of around 1000 to 2000 kms, a metallic "surface" of hydrogen will form in these bodies, one capable of conducting heat and electricity in the same manner as all other metallic substances. The existence of this phenomenon is important to the reconciliation of fusion processes, both "hot" and "cold" at TL–9. Cold Fusion is in a solid state, where a metallic catalyst creates a "crystal" of hydrogen that fuses into helium. Further surveys of solar "hot" fusion, where a high pressure solution of metallic hydrogen is mixed with metallic catalysts, is virtually no different than cold fusion, except for the scale. All working fusion reactors that have ever existed are high pressure gas-metal catalytic cells which compress power plant hydrogen to a metallic state, thereby creating a fusion process that can be utilized for useful power.