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Abstract

According to theoretical predictions, insulating molecular hydrogen dissociates and transforms into an atomic metal at pressures P≈ 370-500 GPa (refs.^1,2,3). In another scenario, the metallization first occurs in the 250-500 GPa pressure range in molecular hydrogen through overlapping of electronic bands^4,5,6,7. The calculations are not accurate enough to predict which option is realized. Here, we show that at a pressure of 350-360 GPa and temperatures <200 K, the hydrogen starts to conduct, and that the temperature dependence of the electrical conductivity is typical of a semimetal. The conductivity, measured up to 440 GPa, increases strongly with pressure. Raman spectra, measured up to 480 GPa, indicate that hydrogen remains a molecular solid at pressures up to 440 GPa, while at higher pressures the Raman signal vanishes, probably indicating further transformation to a good molecular metal or to an atomic state.