An Investigation Project Spanning Over 60 Years—Diver Akira Kubo Reveals What Makes the Ryusendo Cave Fascinating and Why He Continues to Take on His Challenge

October 2, 2023

Source: Iwaizumi Town Ryusendo Cave Office

Many of the limestone caves that exist in Japan have not been fully explored. One of them is the Ryusendo Cave, located in Iwaizumi, Iwate. Diving surveys at Ryusendo Cave began in the 1960s, and after overcoming many twists and turns, including interruptions and postponements, researchers are gradually approaching toward grasping a full picture of the cave. This year, the survey is scheduled to resume for the first time in about six years. We asked Akira Kubo, the leader of the survey team, on what makes Ryusendo Cave fascinating, along with his thoughts and enthusiasm for the survey.

Text: with ORIENT STAR editorial team



Speechlessly magnificent, incomparably beautiful


About 60 years ago in the 1960s, a time of postwar economic recovery, the eyes of the world turned to the oceans. It was around this time that Jacques-Yves Cousteau, the inventor of the revolutionary regulator (Aqua-Lung) diving equipment, and others actively attempted to venture out into the ocean.

To support the activities of these divers, major European and Japanese watchmakers developed diving and diving-themed watches. These models became the prototypes for the divers’ watches of today. Orient was no exception and released its first hand-wound models with the names “Diver” in 1964: the Olympia Calendar Diver (known as the “First Diver”) and the automatic Calendar Auto Orient (known as the “Second Diver”). These models were not suitable for professionals as they had water resistance of 40 metres, but five years later in 1969, the Orient King Diver 1000 was released with improved water resistance of 1,000 metres. This marked the beginning of the history of the Orient brand’s authentic mechanical divers’ watches.

The hand-wound Olympia Calendar Diver released in 1964. Because it was designed for daily use rather than for diving, the model also embodied an elegant impression despite its name. In 2021, a revival of this model, the Diver 1964, was released with 200m water resistance.


In 1962, two years before Orient, which was based in Hino, Tokyo at the time, launched its first divers’ model, a diving survey started in Iwaizumi, Iwate, more than 500 km from Tokyo. It surveyed the Ryusendo Cave, which is considered one of the three greatest limestone caves in Japan, but with the full extent of its existence yet to be uncovered. Several years of underwater research by some of Japan's best divers yielded many results, including the discovery of underground lakes—a third underground lake currently made available for tours, and a fourth underground lake closed to the public—but tragedy suddenly struck. In 1968, a member of the survey team lost their life in an accident while diving. The Ryusendo Cave diving survey project was suspended indefinitely, and its full extent was left unknown.

The situation changed 41 years later, in 2009. Diving surveys at Ryusendo Cave were decided to be resumed.

“When I first heard about it, it left me half excited and half anxious. I wanted to go where no one had explored before, but meanwhile, there was the fact that there had been an accident in the past. At the time, I only knew the name Ryusendo, and had no further knowledge of the limestone cave. Thinking it would be better to talk to someone who had dived before about this, I asked Jiro Suga, who had dived there for a TV programme filming, to tell me about his experience.”

This is what Akira Kubo, a diver who has served as the overall director of the survey team since its first resumed survey to date, told us as he recalled his memories. Kubo majored in international environmental policy at a graduate school in the US while also studying risk management for diving. After returning to Japan, he continued to promote safe diving, which led to his position as the leader of the Ryusendo Cave diving survey team.

Akira Kubo: Born in 1953, Kubo is the president of DIR-TECH Divers’ Institute. He balances his time between Tokyo and the Philippines as a diving instructor. He is also a member of the Japanese Academy of Underwater Sciences and the Speleological Research Institute of Japan.


“I was contacted at the beginning of 2009, and was able to actually dive at the end of 2009 after preparations and filing applications to the government office. The day before the survey, the entire team went to the Ujigami shrine in Iwaizumi to pray for safety, and then we went to the third underground lake, where we actually dived. There, we sprinkled salt and sake and prayed for safety once again. When the divers returned safely from their diving expedition that year and reported that they had confirmed the existence of a new underground lake, I was excited upon seeing the images. It is still fresh in my memory.”

Starting with the first survey in 2009, Mr. Kubo has conducted a total of six surveys. He says that the more he learns about the Ryusendo Cave, the more he finds himself fascinated.

“When you stand in the third underground lake, you can only see a hole of about 5m to 6m in diameter, but when you dive down, you see that the hole drastically drops vertically like an elevator shaft to a depth of 33m. It then becomes a slope from there, and after further diving to about 45m, we arrive at a vast area that we call the Natural Bridge. The view that spreads out from a narrow place like an elevator dynamically all at once is grandiose and so magnificent that it leaves you speechless. Another is the beauty of the water. Divers dive with powerful lights for illumination inside caves, and in both oceans and lakes, you usually see reflections of debris. But nothing is reflected in Ryusendo Cave. Even if it does, it is either air bubbles that you generated or debris that fell off from the side walls. I have never seen clearer water.”



Speechlessly magnificent, incomparably beautiful


Diving surveys at the Ryusendo Cave resumed in 2009, but was forced to be suspended again in 2016. This was because the town of Iwaizumi suffered damage from a large typhoon, and restoration of the local area became an urgent priority. Now that the restoration work is almost complete, a preliminary diving survey is scheduled for early summer of 2022 before resuming the project.

“In a sense, this survey is positioned as a kind of preparation, and a full-scale survey will be conducted after the 8th survey. The main purpose of this series of investigations is to understand the entirety of Ryusendo Cave. The Ryusendo Cave is pretty extensive. Initially, we were going along the route that was created by the 1960s survey team. They followed this route and recorded the presence of a large underground lake after passing through a vertical hole. That’s where the accident happened. But it has been discovered that it is apparently impossible to reach the surface by that route. So now, we are creating another route to proceed with the survey, which will be completed in a few more rounds of surveys. In the process, the main task will also be to survey the water and stalactites, and to recover remnants from the 1960s survey team.”

A picture from an actual diving survey. The total amount of equipment worn when diving is said to exceed 100 kg. In the foreground, the diver in the green dry suit is Kubo (photo by Akira Kubo).


Although the surveys to date have been completed without accidents, diving surveys always have risks. In addition, while a portion of the survey costs are covered by the town of Iwaizumi, Kubo and the diving team pay for any shortfalls from their own pockets. Why does the team continue to take on the challenge of investigating Ryusendo Cave, despite the time, costs and even risks involved?

“For example, when you are visiting somewhere for the first time, wouldn’t you want to get up early in the morning and stroll around? You would want to know about the townscape, what kind of scenery you can see, and what kind of people are living and working there, right? I think that kind of curiosity is what drives people. According to a commonly accepted theory, we humans are the kind of creatures that originated in East Africa, migrated to the Arabian Peninsula, crossed the scorching deserts to India, crossed the Himalayas to China, and then crossed the Bering Strait to the Americas. I believe that such DNA is present in everyone, whether strong or weak. I have this strong inside me. You might call it intellectual curiosity, which is the desire to see a different world. People have been motivated by such things, and history has been driven by them, sometimes taking strange turns, but we have managed to survive. We may be on the brink of a real crisis of survival for mankind, but I believe that humans will still find some kind of solution. I don’t know if that means moving to space, to the bottom of the ocean, or some other way, but I personally am fundamentally inclined to believe in the wisdom of mankind. Some part of this might come from my carefree personality (laughs), but I am the type of person who can only think positively.”

Kubo started diving at the age of 21 and studied at the Monterey Institute of International Studies in the US when he was 46. While majoring in international environmental policy, he also studied diving safety and became a leading diver in this field in Japan.


It seems that his intellectual curiosity of wanting to see and know is the driving force behind his research into the Ryusendo Cave. Lastly, when we asked him about his watch, he told us about his “personal rules.”

“While I primarily use a dive computer when diving, I always wear a divers’ watch in everyday life. And even though I wear the watch on land, it has to be a proper divers’ watch with specifications that can be used when diving. The reason is that I want to be reminded on a daily basis that I am a professional diver who works underwater.”

Advanced specifications and powerful design are of course what make divers’ watches appealing, but perhaps the joys of wearing a divers’ watch may also be about being able to feel the adventurous spirit and the romanticism of taking on the unknown that divers like Mr. Kubo possess, and the positive mindset the watch can provide.

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