The same mechanism that lets a diamond stay in its metastable state when the temperature drops and the pressure is let up and the thing stays a diamond instead of going back to graphite, until you raise the temperature above the critical point again, of course.

Metastable states are tricky that way.

Sometimes the critical temperature is well *below* room temperature. Sometimes they are several thousand degrees Kelvin.

Ya dinna knooo, till you try it laddee!

They haven’t dropped the pressure yet. I assume they’ll try that first. They haven’t raised the temperature yet.

They may be just a wee bit nervous!

If this is indeed true in all of its aspects, what mechanism is proposed that would allow metallic hydrogen under such intense pressure to remain in its metallic state sans the pressure?