## 6 Conclusions  
This article analyzed UHS studies collected from scientific databases (i.e., Scopus and Web of Science). Bibliometric tools were used to assess the collected data, allowing for the recognition of research trends and keyword maps. The evolution of the number of publications shows a linear trend that changes exponentially from 2015 onwards. The keyword map shows four clusters: the first two focus on academic research into geological structures for H2 storage, while the latter two center around broader, applied themes highlighting the integration of hydrogen storage into energy transition strategies and industrial infrastructure. Current research trends emphasize the importance of understanding hydrogen behavior in geological structures, the gas cushion used for safety in storage, and the exploitation of hydrogen as an energy carrier. The present study underscores the importance of the geological characterization of structures with potential to be considered UHS sites and, notably, the relevance of the rock/gas/fluid system interactions (geochemical reactions, microbial activities, geomechanical considerations). Laboratory tests and analyses that replicate real injection and storage conditions play a fundamental role in this research. The approach employed in laboratory experiments, whether storing pure hydrogen or hydrogen gas mixtures, in different loading cycles and on rocks from the main formations of interest, significantly contributes to the development of pilot and industrial projects. While most research and development efforts, both at the laboratory and field scale, have been concentrated in developed regions like the United States, Canada, and Europe, it is crucial to expand these studies globally to comprehensively address the pressing issue of global warming. Conducting a preliminary evaluation at the laboratory scale is a crucial step to identify potential changes or developments in storage behavior. However, it is also important to conduct a preliminary site assessment, taking into account aspects such as the geological material of interest, the depth of the structure, the presence of fluids, the type of gas to be injected (pure hydrogen or mixtures), and other criteria that will serve as the basis for experimental design. Laboratory tests serve as the starting point, along with pilot projects, for subsequent large-scale implementation of this alternative in the field. In summary, acquiring knowledge and experiences from laboratory tests, as well as from the various stages of research in underground hydrogen storage, plays a fundamental role in applying this alternative for global greenhouse gas control and efficient energy management. It is imperative to continue researching this technology through rigorous scientific methods to assess its feasibility, address potential challenges, and develop comprehensive solutions that contribute to a more sustainable future. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app142311286/s1](https://www.mdpi.com/article/10.3390/app142311286/s1)]([https://www.mdpi.com/article/10.3390/app142311286/s1](https://www.mdpi.com/article/10.3390/app142311286/s1)). Figure S1: Contribution by country (% relative to the total number of documents); Figure S2. Evolution of the number of publications in the top five countries; Figure S3. Relevant keyword dynamics per year; Figure S4. Main keywords related to cushion gas; Figure S5. Main keywords in the topics. (A) Geochemical reactions. (B) Microbial activities. (C) Geomechanical reactions; Table S1. Flow and batch experiments on different geological structures and rocks; LINK to the original database of publications used in the bibliometric analysis. Conceptualization, E.B.