Risks assessment for large-scale hydrogen underground storages (HUS) is one of the central issues in the gaseous fuel diversification agenda of the European Union’s (EU) energy transition because hydrogen is regarded as a fuel and energy carrier, that will gradually replace natural gas in almost all segments of national economies of the EU Member States. Hydrogen can be stored in the aboveground storage sites, mixed with the natural gas in the already existing gas network, with the help of power-to-gas technology, or underground in deep geological structures. HUS facilities can be distinguished from other types of energy storage primarily by the large volume of gas stored, the seasonal or long storage period, and large capital expenditures related to their construction and exploitation, as their economic viability is directly affected by the cost of electricity for water electrolysis and the construction costs. For HUS, different types of storage – namely, salt caverns, deep water aquifers, and depleted oil and natural gas reservoirs have been proposed, which are currently under scientific scrutiny. Moreover, hydrogen is a specific gas, which needs to be compressed to occupy the smallest possible volume. In comparison with other gases, it has the lowest molecular weight (2.016 g/mol), a low density (0.08375 kg/m3), very low solubility in water, and low dynamic viscosity (0.88 at 200C, 10-5 Pa S). Also, hydrogen has a high mass-energy density (120 MJ/kg) with a very low volumetric energy density (0.01079 MJ/L). These parameters affect underground storages differently, for example, low viscosity of hydrogen and the associated rate of gas movement carry a higher risk of leakage. It should also be highlighted, that HUS have specific environments characterized by potentially high microbial abundance and activity. Subsurface microorganisms can use hydrogen in their metabolism and thus lead to a variety of undesired side effects or risks, such as hydrogen loss, methane, and acid formation, as well as clogging and corrosion. In contrast to aboveground storage systems, HUS have several strong advantages, too: they allow gas to be stored in large quantities, thus reducing storage costs, ensure storage safety – underground storage is less vulnerable to fire or terrorist attack, optimize the land use – traditional aboveground storages may occupy significant areas, which might be used for other purposes, and existing engineering and safety management experience in underground gas storage sector (regarding the storage of the natural gas).