Illustration: Alan Stonebraker, adapted from T. Strobel et al., Phys. Rev. Lett. (2009)

Figure 1: (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 $8GPa$ 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).