South Daito Island, also called Minamidaito Island, is a remote island 400km east of Okinawa Prefecture in Japan. It is an isolated island in the middle of the ocean with a circumference of 400 km and no land. The sea around the island is extremely deep, reaching a depth of 1,000 meters about 2 km offshore.
It is a small island with a population of about 1,200, an area of 30.53 square kilometers, and a circumference of 21.2 km.
It has its own ecosystem, partly because it was unexplored until the 20th century, when it was only occasionally visited by drifters.
Minamidaito Island is a raised atoll island, made of limestone, with over 100 caves and over 20 underwater limestone caves on the island.
Almost all of the caves on Minamidaito Island are located on private land. The water in the underground lake is used for agricultural purposes, and as a rule, entry into the caves is prohibeted.
We negotiated individually with each local cave landowner to explore the underwater caves on the island.
The height from the surface to the water is approximately 15 to 20 meters in most caves.
Most caves do not require SRT tecniques to reach the water surface.
The water of the underground lake is extremely beautiful. The water has almost no flow, and as a result, most of the stalactites in the water remain undestroyed, including the straws.
The water level of the underground lake ebbs and flows, lagging behind changes in sea level by about two hours.
The largest area is about 10 meters deep, and the maximum depth is about 20 meters. At a depth of 7 to 9 meters, there is a layer that borders the freshwater and saltwater. In other words, the upper layer is freshwater.
Minamidaito Island is a sugarcane production area, and red soil flows into the underground lake.
The red soil of Minami-Daito Island is extremely clay-like and siltier than any other caves we have explored in the world.
In our experience, two weeks are needed in this area for the percolation that has fallen and the silt that has rolled up to settle.
In some caves, the path is blocked by silt immediately after entering the water, and its removal is a challenge for us.
We began our research and exploration of the site in February 2023. In the course of that investigation, we surveyed more than 100 caves and dolines and discovered more than 20 subterranean lakes, all of which we dived more than once. The warm cooperation of the people living on Minami Daito Island was indispensable in this research.
We were amazed at the clarity of the water, the extremely well-developed stalactites in the water and their unbroken remains, and the clarity of the stalactites, especially in areas far from the cave entrance.
Because of the lack of flow, the caves did not appear to be long in many cases, as the current did not cut them down and widen them.
However, there are caves that exceed 300 meters on the main line and 400 meters on the half-surface but beyond that we have not yet explored.
We have been proceeding slowly with the survey so far, but now that we have procured equipment, including breathing gas, and made arrangements for coordination with the site, we plan to proceed from here with mapping, additional surveys, silt removal, and surveys from the seaward side.
A major goal of our research is to conduct a survey of the underground lakes in the caves to better resource the water in the underground lakes.
For example, a percentage of agricultural water extraction from underground lakes is done by boring, but the boring is done without knowing how the underground water veins run.
It is hoped that our mapping of underwater caves will enable efficient water extraction.
We have received requests from local farmers to collect water from large holes like the one in the second half of the video, rather than from narrow passages like the first one in the video above.
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