Hydrogen (H2) is considered a clean energy fuel that can provide a sustainable energy market and overcome intermittent production issues because it can facilitate the storage of large quantities of energy to balance out long periods of poor wind power supply and seasonal fluctuations. A vast expansion of the H2 economy requires a massive storage capacity which is available in geological storages such as deep aquifers, salt caverns and depleted hydrocarbons reservoirs. However, Underground Hydrogen Storage (UHS) is a complex procedure where containment security, pore-scale phenomena, and large-scale storage capacity can be influenced by H2 contamination due to the mixing with cushion gases and reservoir fluids.