## 2 Bibliometric Analysis  
In-depth examination and review of predominant influencing factors and mechanisms were undertaken to facilitate the interpretation of UHS within different geological settings. The methodology for reviewing UHS consists of several comprehensive steps. First, a literature review is carried out to obtain existing knowledge on UHS by searching scientific databases for relevant keywords. This includes reading articles, reports, and case studies from prominent journals and industry magazines to describe key geological formations, contemporary technologies, and safety concerns. Next, a geological assessment assesses Lithuania's geological formations, focusing on parameters such as porosity, permeability, caprock integrity, and formation depth, utilizing data from national geological surveys, academic publications, and industry reports. Figure 1 illustrates the integrated methodology for conducting a comprehensive literature review, global bibliometric analysis, and regional assessment of Underground Hydrogen Storage (UHS), with a focus on Lithuania.  
Here, bibliometric analysis is used to quantitatively measure the research impact of the UHS study field [67]. The data from the Scopus and Web of Sciences database are taken from the years 1988-2024 for the bibliometric analysis. With the search keyword \"Underground Hydrogen Storage\" in the Scopus database, 140 research papers were considered for the  
Figure 1: The methodology adopted for the literature review, bibliometric analysis, and regional assessment of UHS.  
analysis. The variation in the number of publications has been shown in Figure 2 from 1988 to 2024. There has been a significant increase in the number of publications since 2020. This indicates a growing interest and attention from researchers in studying and contributing to the field of UHS.  
The analysis of publication patterns across the countries covered in the identified articles offers valuable insights into the topic. The visual representation employs varying sizes of publication icons, corresponding to the volume of publications on UHS studies. Figure 3 shows that the United States leads in research contributions, followed closely by China and Australia. These countries are at the forefront of UHS research, reflecting their strategic focus on sustainable energy technologies. Other significant contributors include Saudi Arabia, the United Kingdom, and Germany, showcasing a global effort to advance this critical area of study. The findings underscore the growing recognition of UHS as a critical component in the transition to a low-carbon energy system, driven by technological advancement, increased investment, and a global push toward decarbonization.  
The representation of a co-occurrence network map illustrating the frequency of specific keywords or phrases co-occurring within a given subject is depicted in Figure 4. This visual depiction proves invaluable in discerning patterns and understanding relationships between diverse concepts or themes within extensive datasets. Notably, through meticulous examination of the co-occurrence map, it becomes evident that research into UHS has spanned various topics. The major keywords used to create the co-occurrence network include \"Hydrogen storage\", \"underground hydrogen storage\", \"salt caverns\", \"storage of hydrogen\", \"hydrogen injection\", \"hydrogen underground storage\", and \"hydrogen recovery\".  
Figure 2: Variation in the number of publications per year on underground hydrogen storage.