I have previously talked about metalic hydrogen, an effect where at high pressure hydrogen begins to act more like a metal. This is in fact one aspect of an ongoing problem in condensed matter physics: What is the phase diagram of hydrogen actually like. This graph would show the changes of hydrogen at different temperatures and pressures as shown below:
It is well accepted that at high pressures of above 500 gigapascals (GPa) that hydrogen can melt at much cooler temperatures than normal although it commonly thought that high pressure causes matter to stay as a solid. It has now been shown that when the pressure approaches 2 to 4 terapascals (TPa) then the melting curve arcs up again meaning that as you might expect that a higher pressure requires a higher temperature to melt. Eventually as pressure increases the hydrogen becomes a Wigner crystal which is a where electrons potential energy vastly outweigh their kinetic energy and so are stuck in an ordered lattice. These results also present an interesting connection between hydrogen and lithium. Normally hydrogen acts in a very similar way to a halogen, being diatomic, lacking one electron from its outer shell, gaseous, forming negative 1 ions and being very reactive. But hydrogen isn’t put above lithium for nothing. Under this pressure both have identical hybridised orbitals leading to the metallic hydrogen. Now it is seen that the melting curve for hydrogen is also very similar so lithium and sodium, further reinforcing its unique place in the periodic table.