The observation of the HPE state of Na-hP4 at pressures above 100 GPa challenges the classic Thomas-Fermi-Dirac (TFD) model, hinting that the physics and chemistry of the elements at extreme conditions has yet to be fully understood . However, sodium is not a highpressure black swan. For instance, among the complex phase transitions of cesium under pressure, the formation of Cs-IV is accompanied by the reorganization and localization of the electron density in interstitial channels, suggesting the formation of a Cs-electride phase . Also Cs-IV is characterized by an unusual bonding mechanism triggered by the hybridization of the 6s and 5d valence orbitals . Na2He, having a fluorite-type structure, was predicted to be thermodynamically stable before being synthesized in a Diamond Anvil Cell (DAC) at P > 113 GPa and T > 1500 K . Calculations have revealed that the valence electrons of sodium in Na2He localize in interstitial pockets, making it, theoretically, another possible HPE. In recent years, other materials have been synthesized and proposed as being possible HPEs, including Ca in its simple cubic (sc) form at ~40 GPa and Mg-sc, which was observed in ramp compression at 1.32 TPa .