At about this time in 1945, Japan cancels a plan to attack the Panama Canal. The Japanese had new submarines—unlike anything the world had ever seen. They’d wanted to use them to destroy canal locks and keep the U.S. Atlantic fleet from traveling to the Pacific.
It was too late. The American presence in the Pacific was already overwhelming, so the Japanese switched their target: They’d aim for U.S. carriers at the Ulithi Atoll instead.
Do you know about the Japanese “super submarine” that was developed towards the end of World War II? That submarine was effectively an underwater aircraft carrier. It had been in the works for years. Originally, the Japanese thought this submarine would be used to attack the American mainland—even including targets as far away as New York City.
In the end, though, the Japanese decided that a single strike on the Panama Canal would be most effective. After all, if they knocked out the Gatun Locks, the canal would be useless for months.
The new submarines were I-400, I-401, and I-402, and they were Imperial Japanese Navy Sentoku-type submarines. The vessels were massive—the largest built during World War II. Each could carry three specially created armed floatplanes, the Aichi M6A1 Seiran.
“No Seiran ever saw combat,” the National Air and Space Museum notes, “but the Seiran/submarine weapons system represents an ingenious blend of aviation and marine technology.”
The wings of the Seiran could rotate and fold back against the main body of the plane, as could the horizontal and vertical stabilizers. The plane folded up so compactly that it could fit into the cylindrical, watertight hangar aboard the submarines. Those hangars also had facilities to heat engine coolant and lubricating oils outside of the planes, eliminating the need to run engines and warm up a plane in advance.
When a Seiran was ready for launch, the Japanese had a complex—but precise and quick—system for “unfolding” the plane as it emerged from the hangar tube. The plane was then launched from a compressed-air catapult mounted on the forward deck.
Finally, detachable floats allowed the plane to land in the water and return to the submarine once its mission was complete. Once there, the submarine was equipped with a special crane to lift the plane back on board.
Reportedly, the Japanese would paint the Seirans to look like U.S. Army Air Forces aircraft, just to confuse matters.
Nevertheless, development of the super-sized submarine and its aircraft proved to be too little, too late. The planned kamikaze attacks, first on Panama, then on Ulithi, never occurred. Instead, the American attacks on Hiroshima and Nagasaki intervened, and Japan surrendered. Its submarines were ordered back to port, but the Seiran aircraft were launched into the Pacific and left behind.
I-400 and I-401 ended up surrendering to Americans on their way back to Japan. The Japanese commander, Tatsunosuke Ariizumi, shot himself soon afterwards. Americans, of course, didn’t quite know what they had captured until they got the submarines to Pearl Harbor for further study.
All three submarines were eventually scuttled to prevent the Soviet Union from gaining access to the technology.
“More time could have been spent documenting them, but there was a Cold War beginning,” James Delgado of the National Oceanic and Atmospheric Administration concluded. “It was important to get those subs on the bottom and keep them out of the hands of the Soviets.”
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Aichi M6A1 Seiran (Clear Sky Storm) (National Air & Space Museum)
Benjamin Brimelow, Japan built monster plane-carrying subs to attack US cities during World War II, but they never fired a shot (Business Insider; Nov. 8, 2020)
Bill Yenne, Panic on the Pacific: How America Prepared for the West Coast Invasion (2016)
Henry Sakaida et al., I-400: Japan's Secret Aircraft-carrying Strike Submarine : Objective Panama Canal (2010)
Louis Lucero II, Finding Japan's Aircraft-Carrier Sub (New York Times; Dec. 2, 2013)
Suzanne Roig, World War Two era Japanese submarine found off Hawaii coast (Reuters; Dec. 3, 2013)
Thomas S. Momiyama, All and Nothing: After Pearl Harbor, the Japanese planned to strike the United States with aircraft borne by giant submarines (Air & Space Mag.; Nov. 2001)