The Power to Gas or P2G principle is a technique to convert the, through renewable energy sources produced, surplus electricity into storable gas mixture of hydrogen and methane. This surplus electricity is created through the seasonal variations of generation and demand. The generated hydrogen methane mixture can be used as energy carrier and stored on a large scale in the underground. Within the scope of this study, a risk assessment of such a P2G facility, which is storing the 10% hydrogen / 90% methane mixture in a depleted gas field in the Molasse Basin in Austria, is provided. Due to the lack of experience and historical data from similar projects, expert judgements of identified risks are used to quantify the data. This procedure is accompanied by high uncertainty. Therefore, Monte Carlo and sensitivity analysis are used to evaluate the data within the created bow-tie models with surface and subsurface related risks. Risk matrices, societal risk charts, spider charts and tornado charts are used to interpret the risks associated with the specific P2G facility. The results show that adding 10% hydrogen to the natural gas only has minor effects on the safety of the facility. In the specific evaluated facility, the only increased risk associated to the addition of hydrogen to the system is microbial activity. This results from hydrogen’s potential as an electron donor within the reservoir. Microbial activities are not affecting the safety of the facility directly. They are resulting in potential financial damage to the company due to biomass accumulation or/and loss of calorific value in the reservoir. Besides subsurface microbial activity, surface risks have been evaluated too. Major identified surface risks are corrosion and human interference. They are both not directly dependent on hydrogen. This enforces the statement that addition of 10% hydrogen to the system does not change the safety of the facility significantly. Furthermore, it is important to state, that the study is not only a risk assessment example on the specific P2G project, which is storing hydrogen-methane mixtures in a depleted gas field in Austria. It is also a comprehensive collection, analysis and evaluation of risks associated with underground hydrogen-methane storage. The study provides a foundation on further, more detailed work in this, still young, field of research. This finally should contribute to further projects related to large scale storage of surplus renewable energy which might increase in relevance in the future.