(Inset) The face-centered-cubic (fcc) based structure of the newly discovered compound SiH4(H2)2, in which 8 out of 9 atoms are hydrogen. The blue spheres indicate SiH4 units (which may not have definite orientations) and the red spheres represent H2 pairs (with also as yet unknown orientational physics). (Raman spectrum, right) SiH4(H2)2 at 8 GPa and an average electron density 50% greater than that of silane shows excitations characteristic of molecular hydrogen (H2) and plausibly connected with the placement of the H2 pairs. (Spectrum, left) The deuterated system SiH4(D2)2 again shows excitations characteristic of the D2 molecule. (Spectrum, middle) The presence of the mixed isotope pair HD means that there is time dependent migration of H from the blue regions into the red regions (and D can be migrated into the blue regions).