DFT calculations have predicted that under extreme pressure magnesium would adopt an fcc phase between 456-756 GPa, and a sh phase at higher pressures, both characterized by large ELF blobs in interstitial sites. Later, ramp-compression experiments provided evidence for four novel non-close-packed structures of Mg between 0.31-1.32 TPa. Though the structures of the lowest-pressure phase could not be unambiguously identified, at 0.56, 0.96 and 1.32 TPa the data was consistent with distorted fcc, sh and sc structures, respectively, and DFT calculations revealed charge accumulation in their interstitial regions. Since these prior studies showed only the computed ELFs, but did not perform a topological analysis of the electron density, we calculated the electronic structure of Mg-sc at 1.3 TPa using the R 2 SCAN functional, confirming the presence of a NNM in the interstitial (Figure ), which integrates ∼1.4 electrons in its basin. Previous DFT calculations predicted that both the fcc and sh structures are metallic, 116 and our meta-GGA calculations predict the same for Mg-sc (and Ca-sc). Thus, the combined experimental and theoretical results suggest that in addition to the alkali metals, the alkaline earths can be stabilized by the relocation of electron density in interstitial sites upon (sometimes intense) compression.