## 1 Introduction  
Recently, as countries worldwide are striving to address the challenges of climate change and global warming by promoting carbon neutrality, leading environmentally friendly nations such as Europe, Japan, and Australia have implemented and announced various policies to activate the hydrogen economy and transition their existing energy infrastructure into eco-friendly energy systems [1, 2]. For example, in 2019, the European Union (EU) announced a plan to expand the supply of hydrogen and fuel cells by 2050 as part of its 'Hydrogen Roadmap', and Australia revitalized the hydrogen economy by establishing a green hydrogen production and supply infrastructure [3]. In Korea, there has been a concerted effort to transition towards a hydrogen economy since the publication of the 'Hydrogen Economy Activation Roadmap' in 2019. The expansion of hydrogen production and supply inevitably requires large-scale and reliable hydrogen storage technologies. Storing large quantities of hydrogen above the ground raises concerns about various safety hazards and explosion risks, as well as challenges in selecting suitable sites for large-scale storage. Therefore, there is growing interest in underground hydrogen storage methods [4]. Underground hydrogen storage is considered to be the most stable and economical method [5]. The Hydrogen Technology Collaboration Programme (TCP), a subsidiary program of the International Energy Agency (IEA), emphasizes the essential role of underground storage facility technologies after 2030 when a significant increase in global hydrogen supply is expected [6]. The adoption of underground hydrogen storage technology is even more essential in countries such as Korea, which has a limited land area and high population density. There are several methods of underground hydrogen storage, including salt caverns, aquifers, depleted oil fields, and rock caverns. Salt cavern storage involves the creation of cavities by dissolving underground salt layers, a method that has been successfully employed in Germany and the United States. It is preferred because of its relatively low construction cost and low risk of hydrogen leakage and contamination [7]. Depleted oil-field storage is a method for storing hydrogen in geological structures that are left vacant after gas and oil extraction. The aquifer storage method stores water using boreholes in an area where a relatively low-permeability cover rock is developed in the upper part of the aquifer and has an economic advantage, as it does not require artificial joint excavation. Rock cavern storage involves artificially excavating cavities in hard rocks and lining them with insulation materials and concrete to ensure stable energy storage. Worldwide, projects and preliminary research related to hydrogen underground storage have primarily focused on the salt cavern storage method [8; 9; 10; 11; 12]. The United States operates three underground salt cavern hydrogen energy storage facilities in Texas, and Utah aims to construct the world's largest salt cavern hydrogen storage facility capable of storing 1000 MWh of hydrogen energy by 2030. Some European countries with rich salt layers, such as Germany and Poland, have evaluated their geological conditions and hydrogen storage potential [13; 14; 15]. France conducted geochemical modeling studies to understand the impact of subsurface microorganisms on hydrogen loss [16]. In addition, studies have been conducted to evaluate the hydrodynamic behavior of the bedrock constituting a geological structure due to the injection of hydrogen [17; 18; 19]. However, it is noteworthy that hydrogen storage facilities utilizing salt caverns are operational in very few countries, and many nations lack the geological conditions necessary for constructing salt caverns (Figure 1). Therefore, there is a need in Korea for a unique underground hydrogen storage system that utilizes concrete protective structures at shallow depths, aligning with the country's geological environment (Figure 2).