The most striking aspect of the high pressure behaviour of carbon is its tendency to rehybridize from sp or sp² bonding towards sp³ bonding. Rehybridization of graphite under high-pressure-high-temperature conditions is well-known and exploited in the commercial synthesis of diamond abrasives. The transformation to the thermodynamically stable cubic diamond phase is greatly aided by the presence of a catalyst, such as iron or nickel. In the absence of catalyst, hexagonal or lonsdaleite diamond is formed 1-31, 2, 3, illustrating the possibilities for synthesis of thermodynamically metastable phases during carbon transformations. We have research interests in the metastable materials formed by compression of solids and molecules containing carbon and hydrogen storage in carbon. Here we review a longstanding unsolved problem, the nature of the sp³ bonded “transparent phase” of carbon 4, as well as speculate regarding the possibilities for exploiting reversible sp²-sp³ bonding transformations in carbon based hydrogen storage materials.