The original version of this Article omitted references to previous experimental reports on solid hydrogen that are relevant for a full understanding of the context of the previous work. The added references are: 47. Akahama, Y. et al. Evidence from x-ray diffraction of orientational ordering in phase III of solid hydrogen at pressures up to 183 GPa. Phys. Rev. B 82, 060101 (2010). 48. Zha, C.-S., Liu, Z. & Hemley, R. J. Synchrotron infrared measurements of dense hydrogen to 360 GPa. Phys. Rev. Lett. 108, 146402 (2012). 49. Dias, R. & Silvera, I. Observation of the Wigner-Huntington transition to metallic hydrogen. Science 355, 715-718 (2017). 50. Eremets, M. I. & Drozdov, A. P. Comments on the claimed observation of the Wigner-Huntington transition to metallic hydrogen. Preprint at http://arxiv.org/abs/1702.05125 (2017). 51. Loubeyre, P., Occelli, F. & Dumas, P. Comment on: "Observation of the Wigner-Huntington transition to metallic hydrogen". Preprint at http://arxiv.org/abs/1702.07192 (2017). 52. Goncharov, A. F. & Struzhkin, V. V. Comment on "Observation of the Wigner-Huntington transition to metallic hydrogen". Science 357, eaam9736 (2017). 53. Liu, X.-D., Dalladay-Simpson, P., Howie, R. T., Li, B. & Gregoryanz, E. Comment on "Observation of the Wigner-Huntington transition to metallic hydrogen". Science 357, eaan2671 (2017). Citations to these reference, plus reference 21, have been added to the fourth sentence of the Introduction: 'The experimental realisation of atomic metallic hydrogen has remained elusive despite intense research efforts lasting over 30 years4-7,21,47-53.' This has been corrected in the PDF and HTML versions of the Article.