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Owing to their extremely high energy density,single-bonded polymeric nitrogen and atomic metallic hydrogen are generally regarded as the ultimate energetic materials.Although their syntheses normally require ultrahigh pressures of several hundred gigapascals(GPa),which prohibit direct materials appli-cation,research on their stability,metastability,and fundamental properties are valuable for seeking extreme energetic materials through alternative synthetic routes.Various crystalline and amorphous polymeric nitrogens have been discovered between 100 and 200 GPa.Metastability at ambient conditions has been demonstrated for some of these phases.Cubic-gauche and black-phosphorus polymorphs of single-bonded nitrogen are two particularly interesting phases.Their large hystereses warrant further application-inspired basic research of nitrogen.In contrast,although metallic hydrogen contains the highest-estimated energy density,its picosecond lifetime and picogram quantity make its practical mate-rial application impossible at present.“Metallic hydrogen” remains a curiosity-driven basic research pur-suit focusing on the pressure-induced evolution of the molecular hydrogen crystal and its electronic band structure from a low-density insulator with a very wide electronic band gap to a semiconductor with a narrow gap to a dense molecular metal and atomic metal and eventually to a previously unknown exotic state of matter.This great experimental challenge is driving relentless advancement in ultrahigh-pressure science and technology.