Sensuikan Toku


Japanese Submarine Fleet

Divers Find Giant World War II Japanese Relic


By James Donahue

April 2005


As an old marine historian, I have always wondered about rumors of a monster submarine/aircraft carrier built and temporarily operated by the Japanese during World War II.


The rumors proved true when deep-sea divers for the University of Hawaii discovered the sunken remains of the Japanese Navy’s 400-foot-long I-401 submarine in the waters off Oahu.


This submarine was unique to say the least. It was among the I-400 Sensuikan Toku class of Japanese submarines, the largest built before the nuclear ballistic missile launching subs devised by U.S. and Russian Navies in the 1960s, and in effect, they were underwater aircraft carriers. Three of these vessels were built before the war was over.


This particular monster sub was especially designed to carry a fleet of four especially developed aircraft to Panama, come to the surface, launch the planes, and in this way bomb the Panama Canal.


Had the Japanese developed this class of submarine and thought of this idea earlier in the war, they might have caused more havoc to America than just the attack on Pearl Harbor. As it was, the submarines were seized along with a fleet of other Japanese vessels at the end of the war.


As the story is told, I-401 and a fleet of other ships were blown up and purposefully sunk in the Pacific Ocean by American warships in 1946. The concept, while it existed and was a working submarine at the time it was captured, was never tried in a real wartime situation.


Little is known about this vessel except that the Sensuikan Toku class of subs were very large, could reach a maximum depth of 330 feet, and they carried a crew of 144.  


Each sub carried three Seiran, or “Mountain Haze” bombers that folded up inside a watertight hanger. They also carried parts to construct a fourth bomber after the first three were launched. The planes could be made ready to fly in minutes. They also had wing floats so they could land along side the submarine after each mission.


When fully loaded with fuel, these subs were designed to travel up to 37,000 miles, or one and a half times around the world.


The Japanese Navy had not only the bombing of the Panama Canal on their minds when they launched the I-400, the I-401 and I-14, authorities learned.


Another special mission, code named Operation PX, involved the use of these aircraft to drop infected rats and insects with bubonic plague, cholera, dengue fever, typhus and other deadly diseases on American West Coast cities. When the bacteriological bombs could not be prepared in time, the target was changed to the Panama Canal.


Truman’s decision to use America’s new super weapon, the atomic bomb, on Japan, stopped both missions in their tracks. A delay of perhaps another week might have made a significant difference.


I-400 and I-401 were captured at sea a week after the Japanese surrendered in 1945. The commander committed suicide and the mission to Panama was never completed.


The three big submarines were taken to Pearl Harbor in late 1945 under the command of an American prize crew. Also brought to Pearl Harbor at that time were I-201 and I-203, two top-secret Imperial Navy submarines said to be twice as fast as American submarines.


After U.S. engineers and military personnel made a thorough study of these vessels, the decision was made to scuttle them for political reasons. It seems that Russian scientists were demanding access to them. Russia was an ally during the war, although Russian communism and American capitalist ideologies could not allow the two nations to co-exist peacefully and both sides knew it.


On May 31, 1946, the four top-secret Japanese submarines were taken out into the Pacific and sunk by torpedoes from the American submarine USS Cabezon.


It lies about 820 meters deep, off the coast of Barbers Point. One diver said the bow is broken off just forward of the aircraft hanger. The two parts are connected by a debris field.


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