Chousei Coal Mine is located in Ube City, Yamaguchi Prefecture, Japan, and is part of the Ube Coal Field.
Ube Coal Field is a group of submarine coal mines that are no longer in operation.
In the late 19th century, mining operations in the Ube coal field began in earnest with the opening of a coal bureau by the Yamaguchi clan, and British engineer Maurice was invited from Nagasaki to operate the mines with the introduction of Western-style coal mining technology.
After the Meiji Restoration, management of the Ube coal mines was transferred to the private sector in 1872, and a number of coal mines were opened during this period as demand increased due to the Sino-Japanese War.
The mining of the seabed was introduced in the Ube Coalfield after 1912, and it is said that the amount of coal produced in the Ube Coalfield doubled after that time.
The Ube coalfield did not continue to operate peacefully, and several major seawater inflows occurred.
Although operations continued to be affected by the increase and decrease in demand due to the war, all coal mines in Ube City closed in 1967 due to the gradual shift to oil as the main energy source after World War II.
The coal mine where the seawater inflow accident occurred could not be rescued or the bodies salvaged, and the entire mine shaft was abandoned.
Therefore, the bodies and relics left in the submerged mine shafts were left untouched.
The chousei coal mine is one of the Ube coal mines that experienced a flooding accident in 1942 that resulted in the deaths of 183 people.
Note that 137 of the 183 victims of the Chousei Coal Mine accident were from the Korean peninsula.
It is reported that there were more Korean workers in the Ube Coal Mine than in other areas of Ube coal field.
It is estimated that about 40% of the total workers in the Ube coalfield were foreign workers, and about 85% of these foreign workers were from the Korean Peninsula.
The existence of coercion against foreign workers and the quality of the work environment are disputed in some cases, but we will not discuss those issues here.
In any case, the fact remains that 183 bodies were left in the submerged coal mine.
Piers are the exhaust and drainage stacks of the tunnels.
The original entrance to the Chousei Coal Mine has been reclaimed and we are attempting to enter the underwater mine from the pier.
Of course, we also have in mind to dig back the original entrance.
The piers protrude about 5 to 7 meters above sea level, and the water surface inside the piers is approximately the same height as sea level, but seems to be slightly higher than sea level.
We began our research and exploration of the site in July 2024.
During the initial survey, we tried to enter the pier of offshore side (called offshore pier), but we were unable to enter the pier due to high waves caused by a typhoon a few days before the survey date, which prevented us from working to build scaffolding to enter the inside of the pier.
During the second survey, we were able to enter the offshore pier of the two piers.
When we went underwater, the transparency deteriorated to about 50 cm from a depth of about 7 meters.
The water inside the pier appeared to be almost entirely freshwater.
Diving through the interior structures, we found a pile of broken pipes and scaffolding at a depth of around 27 meters.
The temperature at the bottom of the water was 18 degrees Celsius, and since the seawater temperature was 22 degrees Celsius at this time, there appeared to be little or no seawater in the mix.
According to the drawings, the entrance to the tunnel was located at that depth, so we searched along the inner wall of the pier but could not find the entrance due to the piles of objects.
The next survey will be conducted in the shore pier where the drawings show no such piping, etc.
If we are still unable to enter the tunnel, there is a possibility that salvage work of the broken piping will be conducted.
We attempted to excavate the entrance to the mine, which had been reclaimed at the time of the accident 80+ years ago.
Fortunately, drawings and other documents from the period when the mine was in operation remain, and the general location of the mine entrance can be determined.
We began excavation work with heavy equipment on September 24.
We were unable to excavate the entrance to the tunnel on the day we began construction, but fortunately again, we were able to find the entrance to the reclaimed tunnel the following day, September 25, 2024.
The cost of these excavations was funded by crowdfunding.
Water trapped for more than 80 years began to overflow from the entrance of the mine.
The frame of the tunnel was made of pine wood, and the entrance section was 2.2 meters wide and 1.6 meters high.
The water surface rises and falls with the ebb and flow, indicating that there is some kind of traffic with the ocean. However, upon confirmation, the water was freshwater, there are no large holes in the sea.
We spent several days there, clearing the ground and taking certain measures to prevent the surrounding soil from sloughing off. We also paved the pathway to the entrance of the mine with concrete.
After the land clearing, a memorial ceremony was held by the civic orgnization on October 26, 2024 with the families and relatives of the victims of the collapsed coal mine.
We conducted our second dive survey at the end of October.
On October 29, we conducted a dive from the pier closer to shore(called shore pier), and on October 30, from the entrance of the mine.
In the previous survey we used an open circuit for simplicity, but this time we used a closed circuit to reduce the risk of collapse because of the high possibility of getting more inside the mine.
This is because it is assumed that the breathing bubbles will contribute to the collapse, and in the long term, the oxygen in the exhaled air will degrade the tunnel frames.
According to a survey conducted in the 1980s, there is a history of being able to descend to the very bottom because there was little piping or other obstructions inside the shore pier, so there was hope that it would be possible to enter the interior of the mine from shore pier in addition to the entrance of the mine.
We prepared our equipment on the morning of the 29th and attempted to dive to pier in the afternoon.
The pier on shore is about 6 meters high, and we had hooked rope ladders to the outer and inner walls in advance so that we could ascend and descend.
The waves were high that day, and it took some time to get to the shore pier, but we reached them safely and succeeded in getting inside the pier.
When inside the shore pier, the water was murkier than offshore pier and did not provide much of a field of view.
The visibility was clear enough to see one's hand down to a depth of about 5 meters, but below that depth, the visibility was 10 cm or less.
We descended to a depth of about 23 m, but could not reach the very bottom because of obstructions by broken steel pipes and other internal structures.
We groped around for a route, but we decided that it would be difficult to continue the survey from the shore pier and decided to cut off the dive after about 10 minutes.
On October 30, a survey was conducted from the entrance to the main tunnel which was thought to offer the best possibility of access to the inside of the mine.
A large number of press gathered to interview the survey on October 30th, we began the dive surrounded by more reporters than ever before.
We had expected the water to be a bit clearer as we went deeper, but the clarity was about 10 cm all the way.
Although there were only a few moments when there was enough visibility to see own hands, we were able to get in to the point where we we used up one of our reels.
Using a different reel, we continued on a little further, and although it looked like we could still go further, we decided to turn back this time because we were about to reach our scheduled dive time.
The depth at which we decided to turn back was 26.1 meters, and we were able to almost reach the very bottom which is estimated to be about 30 meters.
In addition, the water temperature was about 18 degrees Celsius, all fresh water, and the slope angle was about 10-15 degrees.
Since we think the skeletal remains are located near the bottom area, we believe that there is a high possibility that we will be able to actually recover the remains next time.
From January 31 to February 2, 2025 we carried out our third exploration.
During this exploration, it was hoped that we would actually find remains.
Although the exploration was conducted during the season of winter, with a sprinkling of snow, the enthusiasm of the press seemed to overpower the winter weather.
The diving began at the entrance of the tunnel under the expectation of the local civil society, the bereaved families, and the press, and broke through the previous point without any problem and proceeded further forward, but then a problem occurred.
At the far end of the tunnel, at the depth of 30m, the timber looked as if a jungle gym had collapsed, and it was impossible to break through that point.
On the first day, we were forced to return to the entrance, and on the second and third days, we tried to break through that point by hand with zero visibility, but in the end we could only assume that the tunnel was causing a collapse in that location.
At this moment, Yoshitaka Isaji, the leader of the diving team, began to think that it might be difficult to reach the far end of the tunnel by diving from the tunnel entrance.
As a result of this expedition, we brought back several man-made objects from inside the tunnel. The items in this photo appear to be wooden equipment used for ventilation inside the tunnel.
This was the fourth exploration between the 1st and 4th of April, although the previous exploration had ended with harsh results.
This time was conducted with Kyungsoo Kim and SU Eun Kim, friends of project leader Yoshitaka Isaji, a South Korean diver who runs Tech Korea.
The objectives of this expedition were 1. to see if it was possible to go further from the point currently reached by another person, and 2. to see if there was a route between the entrance of the tunnel and the collapse point to move to a side road.
The route to move to the side road did not exist on official maps near the entrance to the mine, but was said to exist according to the testimony of those who worked there, and we needed to be sure anyway.
On the first and second days, we checked whether it was possible to pass through the collapse point, but visibility was poor and we were unable to do so.
This time, we also conducted surveys using MNemo, which allowed us to clarify how far we had progressed in our previous explorations.
On the third day, we carefully checked whether there was a route to move to the side passage between the entrance of the tunnel and the collapse point, but we could not confirm its existence. We had no choice but to conclude that it was impossible to dive into the far area from the original entrance, although there was a slight possibility.
During this expedition, we attempted to use a wired muddy water camera to capture clearer video footage, but the water was so murky that even with the muddy water camera, we couldn't see anything.
During the previous exploration, the underwater exploration leader, Yoshitaka Isaji, had concluded that diving from Pier rather than from the original entrance of the coal mine was the best course of action, and had begun the task of removing the lumber and steel that blocked the passage to the bottom of Offshore pier.
Although we were unable to remove all obstacles inside Pier before this expedition, on the fourth day we dived inside Pier to check its internal structure.
We have set the date for our next exploration in mid-June and have decided to remove the obstacles inside the Offshore pier before then.
As mentioned previously, steel pipes and wood are piled up inside the peya, blocking the way forward.
However, it is expected that there is a path leading to the tunnel at the bottom of the pier (on the map, it was thought that the old tunnel along the main tunnel could be accessed from the Pier), and it was considered that removing those obstacles would allow access to the tunnel.
The removal work was mainly carried out by hand by the local diving shop (VOX plus) and collaborated divers.
We cut through the obstacles with an underwater reciprocating saw and continued to pull them up. Obviously, steel is extremely heavy, and the wood, which had absorbed water, was heavier than we could have imagined.
Some steel materials could not be lifted by hand, and accidents occurred during the lifting operation in which a steel pipe fell from above. Therefore, it was decided to use a crane barge to lift some of the steel materials.
The work on this crane barge cost 2 million yen (approximately US$140,000) per day, and due to budget constraints, we were only able to use the crane barge for one day. Nevertheless, this work enabled us to continue removing obstacles manually.
I know I'm being repetitive, but all the costs for the exploration and removal work so far have been covered by crowdfunding and donations.
The removal of obstacles took more than a month, but finally, a sloping shaft was found at a depth of 32 meters.
The diver in charge of removing the internal structures slipped through the jungle gym-like overlapping structures and discovered a side tunnel leading to the mine shaft.
Despite the risk of falling structures above, the divers successfully tied a rope from the surface to the 32-meter entrance.
The diver in charge of the removal work advanced about 10 meters into the inclined shaft and confirmed that it was not filled in, that it continued further, and the general structure of the entire shaft.
The diver decided that it was too dangerous to continue on his own, and the exploration team took over the search.
This meant that the expedition scheduled for mid-June could now be looked forward to with great anticipation.
We welcomed the days of exploration on June 18 and 19 with high expectations.
Now, we may finally be able to successfully enter the mine passage.
The diving depth was set at 35 meters, which was deeper than expected,and although it may seem excessive, we prepared trimix for diluent for safety reasons.
We prepared oxygen at the 6 meter point on the decent rope inside the pier, and during these operations, there was a problem in which the support diver dropped the reel, but we decided to continue using a spare reel.
The lower part of the Pier was about 10cm transparent, but we successfully reached the inclined shaft by groping our way, passed through the restriction between the end of the inclined shaft and the room-like space, then reached what appeared to be an old mine passage beyond the room-like space.
Once we entered the old passage, visibility improved a little, and we could see about 50 cm to 1 meter ahead.
The old mine tunnel ran in the direction shown on the map, so we decided to follow the passage further into the mine.
Visibility of one meter was the best we could get in this coal mine, and we quickly advanced about 120 meters and use up our reel and turn back.
It was very regrettable that I lost my reel as soon as I started diving, but we secured a route into the tunnel and confirmed that it continued on, raising our expectations for the next day.
On the second day of exploration, we easily reached the point we had reached last time (of course visibility was zero up to that point), connected the explore reel to the line end, and began to advance further.
In order to limit myself, we only brought a reel that was 200 meters long, because the total length, including 100m already extended in the previous dive, is approximately 300 meters, but this was calculated as the maximum distance that could be safely returned with only two tanks in the event of a bailout.
When we passed the point we had reached last time, visibility was about one meter, but after advancing about 50 meters, there was a moment when visibility dramatically improved.
The old mine shaft gradually slopes downward, reaching a depth of 40 meters, below which is a layer of saltwater (to be precise, the fact is that the salinity is higher than at shallower depths, so it is unclear whether it is completely seawater or not).
The tunnel reaches a depth of 42 meters and then becomes completely horizontal, with the interior of the tunnel almost completely filled with seawater.
The saltwater layer has a visibility of more than 5 meters, and no floating objects could be seen in the water.
We moved forward, feeling both happy that the visibility had improved and concerned that the water was 10 meters deeper than we had expected.
As visibility improved, a gate-like structure quickly came into view.
We attempted to pass through it several times, but the passage was too narrow, so we decided to remove one tank to pass through the gate since we were concerned that forcing our way through would damage the structure and cause it to collapse.
There was a thin pillar on the inside of the gate, we don't want to stimulating it, but we tied a line to it since there was no other option, and we moved forward.
Due to the high transparency, the exploration progressed at an unprecedented speed, and a new gate appeared, but again, we broke through the narrow passageway.
As we continued further, a fork in the road appeared.
A fork in the road! We felt a sense of accomplishment at having successfully entered the inner part of the coal mine. It is truly regrettable that the battery of the action camera used for filming ran out just before reaching this point.
The path split into two, and I decided to check the right path first.
Swimming far down the right passage, I came across a brick wall, but it ended in a dead end after about 70 meters.
Upon further reflection, I realized that this passageway was a dead end on the floor map, so I decided to proceed in the opposite direction. Perhaps I was a little nervous, which is why I didn't notice it at first.
I started moving toward the opposite side and advanced about 20 meters, but my TTS (Time to Surface) reached about 50 minutes.
Δ+5 is about +8, so I decided that if I returned now, the decompression time would be about 100 minutes, and decided to return at this point.
The total dive time was likely to be around 200 minutes, and since we had said that the planned dive time would be around 120 minutes, we thought it would not be a good idea to extend it any further. In any case, there was not much reel left, so it would not have made much difference.
This dive resulted in a total dive time of approximately 200 minutes, with 100 minutes at the bottom and 100 minutes of decompression time.
After finishing the dive, we left Peya and headed to the beach to report the results to the media.
I think we can finally say that “we were able to enter the mine” this time.
For our next expedition, we plan to install bailout tanks in the mine shaft, and then carry out a plan with a bottom time of 150 minutes and a decompression time of 215 minutes.
The day when we will finally be able to recover the remains may be approaching. We eagerly await our next expedition.
Copyright © 2024 Japan Karst & Underwater Exploration Project