<p>Underground hydrogen storage (UHS) is rapidly being identified as a key technology for the large-scale implementation of hydrogen in future low-carbon energy systems. The efficiency and safety of UHS are significantly influenced by complex coupled processes of rock-fluid interactions, reactive transport, and long-term reservoir integrity. This research aims to offer a comprehensive amalgamation of the current state of knowledge on hydrogen storage in geological formations, with a focus on hydrogen-brine-rock-microbe interactions under realistic reservoir conditions. Special emphasis is given to the influence of mineral composition, temperature, salinity, and redox conditions on abiotic geochemical reactions, microbial hydrogen oxidation, and associated fluid chemistry changes. The quantified evaluation of parameters such as capillary entry pressure (0.5–10&#xa0;MPa), Biot coefficient (0.65-1.0), interfacial tension (50–72 mN/m), and contact angle (20–60 degrees) establishes critical limits related to hydrogen containment and possible leakage. These parameters play a critical role in determining the development of effective stress, as well as capillary sealing and flow behavior, which are modulated by wettability. These parameters thus establish safety limits for subsurface hydrogen storage. Through the combination of geochemical, microbiological, and reservoir engineering approaches, this study points out the important uncertainties, monitoring requirements, and research needs for ensuring the safe, efficient, and long-term storage of hydrogen in underground geological formations.</p> Graphical Abstract <p></p>