Akiyoshidai is Japan's largest karst plateau, spreading across the central and eastern parts of Mine City, Yamaguchi Prefecture. The limestone distribution covers approximately 93 km2, making it unparalleled in scale within Japan.
More than 400 limestone caves have been confirmed in Akiyoshidai. It is also home to Akiyoshido (Akiyoshi Cave), one of Japan's three major limestone caves and a Special Natural Monument.
Akiyoshidai is divided by the Higashikawa River into two plateaus: the East Plateau (Higashidai) and the West Plateau (Nishidai). The East Plateau (Akiyoshidai in the narrower sense) was designated as a Quasi-National Park in 1955 (Akiyoshidai Quasi-National Park) and became a Special Natural Monument in 1964. Individual caves have also been designated as Natural Monuments, and their conservation and management are handled with great care.
The limestone that forms Akiyoshidai is believed to have begun forming around 340 million years ago, later uplifting to the surface several million years ago. Over long periods, dissolution by water formed underground water systems, creating complex subterranean waterways that are thought to spread like a network beneath the plateau.
Research on Akiyoshidai's underground water system dates back to the 1950s, with extensive investigations particularly in the 1980s. Some of these studies involved actual diving; however, due to the limitations of cave diving techniques and survey methods at the time, it cannot be said that detailed underwater mapping, systematic investigation, or research on the underwater environment was fully achieved.
※ The entrance of akiyoshido
Underwater investigations in the East Plateau—especially around Akiyoshido—have progressed relatively well. The East Plateau’s underground water system is broadly classified into two systems: the northern Kanoide System and the southern Akiyoshido System, with estimated catchment areas of 1,200 hectares and 1,400 hectares, respectively.
In Akiyoshido, both dry and underwater investigations have been conducted intermittently, and as of December 2017 the total surveyed length (including dry and submerged sections) reached 10,700 meters. Around 30% of this is a submerged section, leaving potential for further exploration of new side passages through diving.
However, considering that (1) surveys were already carried out to a certain extent in the 1970s–1990s, and (2) Akiyoshido is the only cave designated as a Special Natural Monument—requiring especially sensitive experience and environmental understanding—this project has decided to begin with the West Plateau, where fewer investigation examples exist.
Compared with the East Plateau, the West Plateau has fewer subterranean lakes, and past research on its underground water systems has been limited. While there are records indicating that some underwater sections were dived in the past, as far as we have investigated, we have not found examples of detailed underwater surveys or on-site underwater research.
The West Plateau’s underground water system can be broadly divided into three hydrological districts:
Within the Ofuku–Beppudai District lies Terayama-no-Ana (Terayama Caev), which has the largest subterranean lake in the West Plateau. This project first examined the possibility of initiating diving surveys with Terayama-no-Ana as the starting point.
Groundwater around Terayama-no-Ana (Terayama Cave) is said to drain from west to east. If this hypothesis is correct, Terayama-no-Ana may be underwater-connected to the downstream Shiramizu-no-Ike(Shiramizu pond). In particular, the groundwater levels at both locations are reported to be approximately 90 meters, and measurements indicate that groundwater level fluctuation due to rainfall is small throughout the year. Under these conditions, it is possible that they are connected by a relatively large underground waterway—and that a passable space for humans may exist.
Previous studies have suggested that downstream erosional processes once acted strongly in this zone, leading to considerable downward development of caves; however, the lower layers are now submerged due to groundwater-level rise associated with alluvial deposition, making direct observation impossible. By conducting diving surveys, it may become possible to directly observe subterranean structures that have only been inferred indirectly until now.
※ Shiramizu pond (White water pond)
※ Underwater lake of Terayama-no-Ana
Prior to full-scale investigation, we planned a preliminary survey at Terayama-no-Ana for mid-to-late February 2026. This plan will be carried out while maintaining communication and coordination with relevant parties, including the Mine City Akiyoshidai Science Museum, landowners, and the Mine City cultural property protection division.
Because Terayama-no-Ana is a cultural property and the plan involves diving, internal review within Mine City was required, and we were able to obtain prospects for implementation.
To reach the subterranean lake of Terayama-no-Ana, it is necessary to proceed from the First Branch Tunnel through the Second Branch Tunnel and onward to the back of the Third Branch Tunnel. However, the Second Branch Tunnel includes elevation differences and narrow passages; past records also report sections requiring crawling. Therefore, for the full-scale phase of this project, we would prefer—if possible—to reach the lake without passing through the Second Branch Tunnel.
On existing maps/drawings, there is an implication that the First Branch Tunnel may connect to the Third Branch Tunnel via a submerged section, and it may also be possible to pass from underwater directly toward the subterranean lake. Because the outcome of this confirmation could significantly affect the entire future plan, the objective of this preliminary survey is to verify this point.
It has also been confirmed that a water surface exists in the Fourth Branch Tunnel. While access requires SRT (Single Rope Technique), depending on project progress, members with SRT skills may also confirm the water surface in the Fourth Branch Tunnel.
This preliminary survey is planned with land support from university exploration clubs and dry cavers who have prior experience entering Terayama-no-Ana.
Akiyoshidai is one of Japan’s representative karst regions, where underwater investigations have been conducted for more than 60 years. Building on the accumulation of research and exploration by predecessors, we believe it is a challenge of great significance—and responsibility—to pursue a clearer overall understanding using the latest diving techniques and survey methods.
Akiyoshidai has also served as a major field for academic research, and contributing to the understanding and study of its underground water systems is central to the aims of this project. Based on the results of the February 2026 preliminary survey, we will expand the investigation area step by step and work toward clarifying the full picture of the Akiyoshidai underground water system.
The outcomes and data obtained will be presented through the Mine City Akiyoshidai Science Museum and the Speleological Society of Japan, and will also be shared with researchers involved with Akiyoshidai in order to support continued investigation and research.
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