Underground Hydrogen Storage (UHS) is a promising technology for large-scale energy storage. Geological formations, in which buoyant hydrocarbons have been retained for a long term, provide vast capacity for hydrogen storage in a safe manner. Different types of formations have been considered, such as salt caverns, saline aquifers, and depleted gas reservoirs. In this study we focus on saline aquifers. Although lessons were learnt from similar studies such as carbon capture and sequestration and underground gas storage, the unique thermodynamic and physical properties of hydrogen make UHS distinguished from the other subsurface storage projects. To this end, we developed a two-phase, three-component reservoir simulator, which incorporated essential physics based on the fully-coupled multi-physics framework of the DARSim simulator. Particularly, properties of fluid mixtures were computed using the thermodynamic model of GERG-2008. In the simulation, different types of cushion gases (CO2, CH4, and N2) were saturated at the top of the aquifer. Hydrogen was then injected into and produced from the aquifer periodically. Results showed that most of the injected hydrogen stayed at the top of the aquifer due to its light density, which led to a high extraction purity. The low solubility of hydrogen was favorable in UHS because dissolution trapping had a negligible impact. Our study confirmed the feasibility of large-scale underground hydrogen storage and showed the impact of different types of cushion gases on UHS.@en