## 3 Overview of UHS Globally and in the Baltics  
UHS technology is still in its early stages in Europe, especially in the Baltic Basin, but there is growing interest and research in this area, indicating potential for its implementation on an industrial scale [68; 69; 70]. Recently, a document published on H\({}_{2}\) production from renewable sources in the Baltic Sea region proposes increasing H\({}_{2}\) production through wind power by 2050, as shown in Figure 5[71]. As H\({}_{2}\) production from renewable sources increases, subsequently we must explore the UHS in the Baltic Basin.  
Figure 4: Co-occurrence network map showcasing the frequency of specific keywords interconnecting within a cluster.  
Figure 3: Distribution of studies conducted across the world on UHS.  
The Baltic Basin encompasses areas in countries such as Poland, Lithuania, Latvia, Estonia, and parts of Russia, as shown in Figure 6. The Baltic basin comprises the Cambrian formation, a rock formation that was deposited during the Cambrian Period, approximately 541 to 485 million years ago. The Baltic Basin has diverse geological formations, including salt caverns, depleted oil and gas reservoirs, and aquifers, all of which have varying potential for UHS, as highlighted in multiple studies [72; 73; 74; 75; 76; 77; 78]. Poland is one of the leading countries in the Baltic Basin actively working on UHS. For instance, [PERSON] et al. [66] reported that out of 27 salt domes in the Polish Lowlands, seven domes best meet the criteria for UHS. Recently, [PERSON] et al. [55] reported that the maximum storage demand for Poland is approximately 35-38 TWh H\({}_{2}\), accounting for approximately 8% of the European capacities. The article states that the average volume and capacity of the caverns in bedded rock salt deposits range from 0.047 to 0.094 TWh H\({}_{2}\), while for salt domes it ranges from 0.06 to 0.20 TWh H\({}_{2}\). The storage capacity in deep aquifers ranges from 0.016 to 4.5 TWh H\({}_{2}\). Furthermore, [PERSON] et al. [79] suggested a system for analyzing and ranking storage traps in saline aquifers in the Polish Lowlands for UHS using a multi-criteria approach. Moreover, [PERSON] et al. [66] identified favorable geological conditions in Poland for UHS in various geological structures, including rock salt deposits, depleted oil and natural gas fields, and deeply seated aquifers. The study suggests that there are specific areas in Poland where these storage options can be realized, such as the Leba Elevation, Fore-Sudetic Monocline, Carpathian Foredeep, and salt stocks in the Polish Lowlands for rock salt deposits, and various oil and natural gas fields in the Polish Lowlands, Carpathians, and Carpathian Foredeep. The study also highlights the potential of the Lower Cretaceous and Lower Jurassic formations in the Polish Lowlands for H\({}_{2}\) storage in deeply seated aquifers.  
In the Baltic Basin region, various H\({}_{2}\) production and storage projects are currently underway to produce 10 million tons and import 10 million tons of renewable H\({}_{2}\) into the EU by 2030. These ongoing projects include the Baltic Sea Hydrogen Collector (BHC), which involves Finland, Sweden, and Germany [80]; the Baltic Sea H\({}_{2}\) project, with the participation of 40 partners from nine nations, including Finland, Estonia, Latvia, Lithuania, Poland, Germany, Denmark, Norway, and Sweden [81]; and the Nordic-Baltic Hydrogen Corridor, where gas transmission system operators from six EU countries collaborate, including Gasgrid Finland (Finland), Elering (Estonia), Conexus Baltic Grid (Latvia), Amber Grid (Lithuania), GAZ-SYSTEM (Poland), and ONTRAS (Germany) [82]. These initiatives  
Figure 5: Thematic map of an optimistic scenario: projected hydrogen production surplus potential in the Baltic area.  
collectively contribute to advancing the use of H\({}_{2}\) as a clean and renewable energy source in the region [83; 84].