Within the framework of energy transition, hydrogen has a great potential as a clean energy carrier. The conversion of electricity into hydrogen for storage and transport is an efficient technological solution, capable of significantly reducing the problem of energy shortage. Underground hydrogen storage (UHS) is the best solution to store the large amount of excess electrical energy arising from the excessive over-production of electricity, with the objective of balancing the irregular and intermittent energy production, typical of renewable sources such as windmills or solar. Earlier studies have demonstrated that UHS should be qualitatively identical to the underground storage of natural gas. Much later, however, it was revealed that UHS is bound to incur peculiar difficulties, as the stored hydrogen is likely to be used by the microorganisms present in the rocks for their metabolism, which may cause significant losses of hydrogen. This paper demonstrates that besides microbial activities, the hydrodynamic behavior of UHS is very unique and different from that of a natural gas storage.