Current Status of Underwater Space Utilization and Challenges to Contribute to a Sustainable Society

The range of human activity has reached approximately the entire land area, leaving little space for development. The outward expansion of human activities has been increasing, and there are attempts to expand the range of human activities to the moon and Mars. In the oceans, floating structures have been widely used as a solution to land expansion and sea level rise in coastal areas. Still, there has been no notable development in underwater space, except for a few experimental facilities built in the 1960s to 1970s. On the other hand, there have been many ideas on the possibilities of using underwater space to realize a sustainable society. Based on pure scientific research, these include the development of seabed and in-water resources, the utilization of deep-sea organisms, and the production of soft materials under extreme environments. Some are based on current social needs, such as renewable energy, food, water, and carbon removal, while others are niche markets, such as highly energy-efficient and stable underwater data centers and leisure/tourism facilities. This paper first reviews the progress of underwater space utilization to date. Literature surveys are conducted on the applications, installation environments, and structure of the undersea facilities constructed around the world; then, the characteristics are summarized. Next, the ideas proposed for the utilization of undersea space are summarized. Based on these results, the applicability of underwater space to current technology and societal needs is discussed from the perspective of contributing to a sustainable society and summarized as issues to be addressed.