Miyako Island is an island of Okinawa Prefecture located about 300 km southwest of Okinawa Main Island. It has an area of approximately 158.5 km² and a population of about 55,000.
The entire island is formed of limestone derived from uplifted coral reefs, and this geological characteristic has created the diverse cave environments unique to Miyako Island.
Because limestone is easily dissolved by rainwater and groundwater, limestone caves form over long periods of time. In coastal areas, caves are also formed by dissolution caused by seawater. As a result, Miyako Island has many caves, both on land and underwater. Some land caves are known as tourist sites, and many underwater caves attract attention from divers in Japan and abroad.
In this project, among these various caves, we focus on the “subterranean lakes” located in the deeper parts of land caves and investigate their underwater environments.
The geological structure of Miyako Island can be broadly divided into the following three layers:
Because the highly permeable Ryukyu Limestone overlies the relatively impermeable Shimajiri Layer, groundwater tends to accumulate near the boundary between them, and subterranean rivers and subterranean lakes are thought to form there.
These strata are not distributed uniformly in horizontal layers. In some places, the Ryukyu Limestone is exposed at the surface, and in other places the thickness of the layers changes significantly.
In addition, faults running roughly north-northwest to south-southeast are developed on Miyako Island, and groundwater is believed to flow along these faults, appearing as springs and subterranean lakes in the deeper parts of caves.
Making use of these geological and groundwater conditions, Miyako Island is home to the world’s first underground dam, which forms the basis of the island’s water resource management.
References on geology and groundwater consulted for this research are listed at the end of this page.
※ 宮古島インギャーマリンガーデン
※ 福里地下ダム
※ 通り池。水中で海とつながる。
It is widely known that Miyako Island and the neighboring islands of Shimoji and Irabu contain many land caves and underwater caves.
Even in recreational diving, Miyako Island is so well known for cave diving that sites such as “Maou no Kyuden” and “Toriike” attract many divers from Japan and abroad.
At the same time, many subterranean lakes in the deeper parts of land caves have a history of being used as domestic water sources, and traces of daily life remain inside the caves.
For this reason, many of these caves and subterranean lakes have been designated as cultural properties or historic sites, and underwater surveys have generally not been carried out.
In that context, Professor Yoshihisa Fujita of Okinawa Prefectural University of Arts, with whom I had previously conducted cave research in other regions, consulted me about investigating the deeper underwater sections of subterranean lakes on Miyako Island that he had not been able to reach because of technical diving limitations.
Investigating unexplored areas that require advanced diving techniques is precisely the kind of challenge I should undertake as an underwater explorer, and after discussion with Professor Fujita, we decided to carry out this research.
At that time, the idea was that the work would proceed “if research funding could be obtained in the following fiscal year.” However, in discussing this, Professor Fujita and I realized that we shared the same concerns about that issue and about the broader way basic research is often conducted in Japan.
※ 伊良部島の「ヌドクビアブ」。戦時中に日本軍が中で過ごした跡が残されている。
1. During the planning of this research, we became aware of two major issues in conventional research methods.
Researchers are highly skilled at evaluating and studying the academic significance of information obtained from research sites. However, in environments such as underwater caves, advanced diving techniques and careful safety management are essential, and this greatly limits both the number of researchers who can dive and the range that can be investigated.
Underwater explorers, on the other hand, possess high-level diving skills and the ability to carry out field investigations, but there are limits to how far they can independently evaluate and communicate the academic significance of what they find.
Therefore, we believe that collaboration between researchers and underwater explorers could produce new research results by combining the strengths of both.
In general, many researchers in Japan have conducted research using funds allocated by their institutions and external research funding such as KAKENHI and other public or private grants.
Institutional research funds are often limited, while external grants have low acceptance rates and require a long time from application to decision. When applications are unsuccessful, repeated reapplications may be necessary, which has also been pointed out as a waste of researchers’ human resources.
This is especially true in biodiversity surveys, where it is impossible to know in advance what organisms may be found in the field. Ideally, joint fieldwork involving taxonomists from multiple disciplines would be desirable, but because of limitations in budget and personnel, such fieldwork is often difficult to carry out.
These two issues can ultimately be summarized as the inability to secure flexible funding for sufficient work in the initial stage.
For that reason, in this project we explored a new framework: to conduct initial surveys efficiently, make the research value visible at an early stage, and then connect that value to funding for the next phase of research.
More specifically, we are considering crowdfunding and small-scale sponsorship from individual supporters and companies by offering, for example:
If this system works, it could make it possible to expand investigations into places that previously could not be studied for budgetary reasons, and it may accelerate future research outcomes.
In this research, we focus on the following three caves:
The subterranean lakes in all of these caves are anchialine environments* and are expected to contain previously undescribed species and other unusual organisms.
*Anchialine environments are special groundwater systems located inland but connected to the sea underground, where freshwater and seawater mix and stratify.
Pottery and bones believed to be human have already been found underwater in Kinsukya-ga, and further archaeological and biological discoveries may be expected in the unexplored sections deeper inside the cave.
Pikyazu is a doline that opens through to the sea. According to a report published in October 2016 in the Bulletin of National Institute of Technology, Sasebo College, No. 53, investigators were able to proceed to a point about 20 meters from the water surface and about 11 meters in depth, and confirmed water flow from deeper inside, but had to suspend the investigation there because of silt accumulation.
In this project, we aim to confirm whether it is possible to reach beyond that point. Regardless of whether that proves possible, we also plan to conduct biological surveys focusing on the mixing zone between the lower-salinity flow coming from deeper inside and the more saline seawater.
※ ピキャズの水面付近
After discussions with Professor Fujita and members of JKUEP (Japan Karst & Underwater Exploration Project), the survey period was set for March 5 to 9, 2026./p>
To address the issues described above, Atsushi Hayashida, who can survey land caves, Yudai Uchida, a cameraman capable of underwater and topside imaging, and archaeology researchers were also scheduled to take part in this research.
As described below, the scope of work was defined in advance. Among these items, underwater cave surveying and 3D mapping are especially important. Although they are fundamental data for academic research, there are still few specialists in Japan with these skills, and we believe the data obtained in this research will make a significant contribution to future studies.
Before the main research, in November 2025 I carried out a preliminary survey with underwater photographer Jun Shimizu, after filing the necessary notifications with Okinawa Prefecture and Miyakojima City
This was done to secure transport routes for diving equipment and to confirm the environment in advance. It may sound excessive, but I believe this kind of preparation is extremely important for efficient and safe research.
*Beautiflu water surface of Kinsukya-ga
※ 友利のあま井の洞窟の内部と水面
On that day, not only Jun Shimizu and I, but also NHK staff who were interested in the research accompanied us. Although the weather was poor, we inspected the research sites and considered routes and logistics.
The water surface of Kinsukya-ga was remarkably beautiful and made me feel that I wanted to dive there immediately.
Of course, its importance is not only aesthetic. Pottery has been confirmed on land, and in the water, pottery as well as bones believed to be human have been found, making it a cave of great academic value.
The water surface of Kinsukya-ga rose and fell with the tides, as expected from prior information, suggesting a connection with the sea inside the spring.
At Amaga of Tomori in Gusukube-Cho, Professor Fujita discovered larvae of marine organisms even though the site is inland.
The presence of larvae of marine organisms strongly suggests that this site too is connected to the sea underwater. Further biological research here may contribute to clarifying how organisms inhabiting such anchialine environments disperse.
In addition, the steps inside Amaga of Tomori, which were installed to draw water, reminded us that this water was once used in daily life.
We also received information about the research sites from staff of the Miyakojima City Board of Education. I would like to record my sincere thanks here.
This preliminary research was covered by NHK Okinawa, Ryukyu Shimpo, and Miyako Mainichi Shimbun.
※ Image by NHK Okinawa 13th Nov 2025
At present, while preparing for the dives, we are refining the details of the research plan.
About two weeks before my planned dive dates, another researcher with whom I have a connection is scheduled to conduct land-based research at the same sites, and I have already received requests such as, “Please check whether there is evidence of tsunami inflow underwater.”
Making it possible for the same place to be studied efficiently from multiple directions at the same time is exactly the framework that this project aims to build, and I am very pleased that we have been able to form a network through which such collaboration is possible.
*Affiliations may not be up to date.
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