  The primary purpose of this paper is to stimulate theoretical predictions of
how to retain metastably hydrogenous materials made at high pressure P on
release to ambient. Ultracondensed metallic hydrogen has been made at 140 GPa
at finite temperatures T in the fluid. The term metallic here means quantum
mechanically degenerate. A single sample of ultracondensed hydrogen has been
made at an estimated pressure of 495 GPa at 5.5 K. Whether that sample is solid
or fluid remains to be determined. Those results imply the long quest for
metallic hydrogen is likely to be concluded in the relatively near future.
Because the quest for metallic hydrogen has been a major driver of high
pressure research for decades, a logical question is whether another research
direction, comparable in scale to that quest, will arise in high pressure
research in the future.
  One possibility is retention of metastable solid metallic hydrogen and other
hydrogenous materials on release of P and T to ambient. If hydrogenous
materials could be retained metastably in the solid on release, those materials
would be a new class of materials for scientific investigations and
technological applications. This paper is a brief review of the synthesis of
metallic hydrogen, potential technological applications of metastable solid
metallic H and other hydrogenous materials at ambient, and published
experimental and theoretical results as general background for what has been
accomplished with metastable phases in the past, which suggests what might be
accomplished in the future.
