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**Advances in Underground Energy Storage for Renewable Energy Sources**  
**[PERSON] \({}^{1,\bullet}\)\({}^{\text{(\textregistered}}\) and [PERSON] \({}^{2}\)**  
The use of fossil fuels (coal, fuel, and natural gas) to generate electricity has been reduced in the European Union during the last few years, involving a significant decrease in greenhouse gas emissions. The global climate goal would be to reach zero emissions in 2050, and a reduction in the last portion of the CO\({}_{2}\) emissions could come from renewables, green hydrogen, and renewable-based electrification. In the current energy transition towards a sustainable economy, large-scale energy storage systems are required to increase the integration of intermittent renewable energies, such as wind and solar photovoltaics. Underground energy storage systems with low environmental impacts using disused subsurface space may be an alternative to provide ancillary services in the European electricity grids. In this Special Issue, advances in underground pumped storage hydropower, compressed air energy storage, and hydrogen energy storage systems are presented as promising solutions to solve the intermittency problems caused by variable renewable energy sources. Nowadays, pumped storage hydropower (PSH) is the most mature large-scale form of storage technology. PHS systems are the primary technology used to provide electricity storage services to the grid, accounting for 161 GW of installed global storage capacity. PHS would need to double, reaching 325 GW in 2050. PSH systems consist of two water reservoirs at different heights. The stored energy depends on the mass of water moved and the net hydraulic head between both upper and lower reservoirs. The round trip energy efficiency is between 0.7-0.8. Topographic limitations in flat areas and environmental impacts currently hinder the development of these systems around the world. Conversely, disused underground space could facilitate the installation of underground pumped storage hydropower (UPSH) systems, where at least one water reservoir is underground. [PERSON] et al. [1] analyzed the economic feasibility of UPSH plants in closed mines providing ancillary services in the Iberian electricity market. Two different options of lower reservoirs were considered: (i) to make use of current mining infrastructure, and (ii) to excavate a new network of tunnels. Secondary regulations, deviation management, and tertiary regulation services considering daily turbine cycle times at full load between 4-10 h were employed to optimize the economic results. Investment costs of 366 MC were obtained when the existing underground infrastructure was used as lower reservoir. Finally, an internal rate of return of 7.10% was estimated to participate in the Iberian ancillary services markets, considering turbine cycle times at a full load of 8 h. Due to the high investment costs, the profitability is reduced whenever a new reservoir has to be drilled. The feasibility study of UPSH plants must also include geomechanical and hydrogeological aspects. [PERSON] et al. [2] studied the geomechanical performance of an underground water reservoir in a closed coal mine. Sandstone and shale rock masses were considered as rock masses to excavate the tunnel networks with a cross-section 30 m\({}^{2}\) and 200 m long. Three-dimensional numerical models were conducted to analyze the deformations and thickness of the plastic zones around the excavations. Systematicground rock bolts and reinforced shotcrete were applied as support systems. The results obtained showed that the excavation of the underground reservoir is technically feasible. [PERSON] et al. [3] carried out a study to determine the impact of hydrogeological features on the performance of an underground pumped storage hydropower plant in Belgium. The subsurface water exchange between the surrounding medium and the lower reservoir of UPSH plants was investigated.