All these systems exhibit intriguing open structures, and calculations find that within them electron density is localized in interstitial regions. Nonetheless, a direct experimental proof of this electron localization remains elusive for high-pressure systems. In fact, the HPE state has only been inferred from first-principles calculations, particularly based on Density Functional Theory (DFT), but, so far, never confirmed experimentally. In fact, preparing and characterizing such materials is inherently challenging, whether this is done with static compression in a DAC or with dynamic compression methods.