RISE has previously studied alternative fuels, such as batteries and gaseous fuels including liquid and compressed hydrogen (GH2). Each fuel has its unique risks. Liquid hydrogen (LH2) is a cryogenic fluid and is thus stored in cooled liquid form, which entails specific risks. The purpose of this report is to, based on the current state of research, map the risks of hydrogen in underground facilities in relation to conventional fuels and investigate which technical measures can be taken to reduce the risks. Unlike diesel, hydrogen (and for instance methane or gasoline) has such a low flash point that an emission can be ignited at normal temperature by a small ignition source. Hydrogen is also very buoyant, with strong diffusion and dispersion characteristics, accordingly it accumulates at high points in a subsurface environment. Hydrogen requires very low energy to ignite at or near stoichiometric mixing with air at around 30%. The lower flammability limit is, compared to other flammable fuel/air mixtures high at around 4%, which means that many smaller releases in ventilated spaces will be too lean. Explosions would require a higher hydrogen concentration, above 8% or more. In subsurface environments, containment contributes to a higher increase in pressure, as well as an increased risk of explosion for both GH2 and LH2. The handling of hydrogen underground can therefore be seen as problematic. When it comes to hydrogen as a vehicle fuel, however, there are safety measures to achieve equivalent safety with conventional vehicles. For example, the shut-off valve (mandatory in regulation) on each tank that reduces the risk of leakage, and through the development of explosion-free composite tanks (not mandatory in regulation) in the event of fire that provide a less dangerous fire scenario than a diesel or gasoline tank in case of fire. When it finally comes to transporting hydrogen, pipelines are the long-term sustainable (and safe) alternative. Transport of compressed hydrogen gives a low amount of gas per trailer and entails relatively higher risks than CNG underground, for example in tunnels. The usage of liquid hydrogen, so far, has an impressive safety record, events like BLEVE or fireballs appear to be rare. The transport of liquid hydrogen provides a larger amount of hydrogen per trailer (than for compressed hydrogen) with a relatively lower risk than, for example, LNG in the open, but a slightly higher risk for explosion of accumulated gas compared to GH2 in enclosed spaces. The safety requirements for transport of compressed hydrogen are less stringent than for road vehicles, e.g., with regard to shut-off valves and melt-fuses and could be improved. Several risk mitigation measures for tunnels and other underground facilities have been identified.