Cs + e -, an ionic compound where the electrons, here found along the chains, act as the anions. In the 1950s it was hypothesized that these structural transitions in compressed Cs were driven by the pressure-induced s → d valence electronic transition, which causes the interatomic distances to become smaller compared to the ranges of the wavefunction, and later this peculiar electron density concentration was rationalized in terms of the overlap of sd hybrids. Since then, many compressed phases have been predicted to be HPEs, including Ca, Al, Mg, 60 C, and compounds such as Na 2 He. The experimental discovery of phases with open structures, which, according to DFT calculations, possess electron density localized in interstitial regions, suggests that the electride state may actually be quite common in compressed matter. However, preparing, characterizing and interrogating HPEs in a DAC or through shock compression experiments, still remains a grand challenge, which has so far precluded the possibility to prove the theoretical predictions of charge localization in the interstitial sites. Moreover, though several advances (to be described in detail below)