Metallic hydrogen is a phase of hydrogen in which it behaves as an electrical conductor. This phase was predicted theoretically in 1935 but has yet to be unambiguously observed, but possibly some new phases of solid hydrogen have been observed under static conditions and electrical insulator to conductor transitions have been reported associated with an increase in optical reflectivity in dense liquid deuterium which is consistent with metallic behaviour. At high pressure, on the order of hundreds of gigapascals, hydrogen might exist as a liquid rather than a solid. Liquid and solid metallic hydrogen is thought to be present in large amounts in the gravitationally compressed interiors of Jupiter, Saturn, and in some extrasolar planets.