Recently a research team from the University of Hawaii at Manoa announced its finding of a formidable Japanese secret weapon sunk 2300 feet below the surface just off Oahu waters.
The secret weapon was the Japanese supersubmarine vessel, the I-400, which was the brainchild of Japanese Admiral Isoroku Yamamoto, who infamously masterminded the December 7th, 1941 attack on Pearl Harbor. Yamamoto was the commander-in-chief of Japanese naval operations in World War II and chief architect of carrier warfare in the Pacific. But his genius did not stop there. His introduction of the I-400 represented a breakthrough tactical change to submarine doctrine and a revolution in submarine design. Historians have stated that modern cruise and ballistic missile submarines would not have been possible without the I-400's pioneering innovations as an underwater platform utilized to attack land targets.
The Sen-Toku class I-400 was innovative not just because of its large size (its 400 ft x 39 ft x 23 ft dimensions dwarfed all other subs of the day) nor because of its range of 37,500 nautical miles (double the range of its Allied contemporaries, and thereby unmatched until the arrival of nuclear subs in the 1960s). Not to mention its displacement of 6560 tons (when submerged) and its ability to be powered by four diesel engines (totaling 7770 horsepower), or four electric engines of 2400 horsepower. Nor its ability to dive to a 328 feet depth on top of its impressive ability to circumnavigate the globe one and a half times without needing to refuel. (To put that into perspective, the Sen-Toku could make three round-trips to the US western coast without refueling, or make one round-trip to any spot on the globe ----- all for the purpose of advancing Japanese military might). Nor even its armament of a 140mm (5.5-inch) deck gun (found abaft the hangar) with a range of 49,000 feet, eight torpedo tubes with twenty torpedo reloads to counter Allied surface vessels at close-combat range, and four water-proofed anti-aircraft autocannons.
Rather, the Sen-Toku class vessels, which could carry 144 to 220 personnel depending on mission, was revolutionary for having 150-foot long, 11-ft diameter watertight hangars built into its hull from which three housed Aichi folding-wing M6A1 Seiran bombers could launch by catapult from the sub's deck. To prevent the hangar's weight from capsizing the sub, the vessel was designed with a twin cylinder hull. Its pressure hull's cross-section had a unique figure-eight shape to afford the vessel the necessary stability and strength to handle the hangar's weight. Plus, along the I-400's centerline was stowage space for the three naval aircraft, with the conning tower offset to port. The foredeck was dominated by the launch catapult, which was angled at an upward slope to promote short take-offs. Strung along the sub's gunwales, from stern to bow, were two parallel sets of demagnetization cables intended to dissipate static charge buildup when the sub's hull sliced through the water, subsequently minimizing chances of hull deterioration. Meanwhile, each Seiran had a top speed of 295 mph and could likewise carry a payload of 18,000 pounds. The naval planes' wings and stabilizers were foldable, and their floats could be taken off and stored separately. Assembly of the Seirans could be performed at night by only four crewmembers, and launching all three Seiran floatplanes would only take 45 minutes. The Seirans, in turn, could land on water and be retrieved with the sub's collapsible hydraulic crane arm. In short, the I-400 was a long-range supersubmarine aircraft carrier with stealthy air strike capability!
Yamamoto's intent for the hybrid supersubmarines was for the Sen-Toku class to serve as weapons surgically striking the US mainland via bombing sorties so as to instill fear on US citizens, thus providing psychological warfare advantage to the Japanese. Moreover, the idea of utilizing germ warfare "bombs" was a strategy being considered, wherein rodents and insects infected with virulent bubonic plague, cholera, dengue fever, and typhoid would be used as a "shock and awe" measure against US coastal targets accessible to the Sen-Toku's Seiran floatplanes. If necessary, the Sen-Toku naval aircraft could even reconnoiter to gather intelligence for other Japanese offensives. The supersubmarines would thereby carry their aircraft underwater to their destinations, surface, launch their planes, then quickly submerge again before being detected. Indeed, with the I-400's unrefueled range of 37,500 nautical miles, the Sen-Toku vessel could have conceivably brought WWII to the continental US shores.
Plans for Yamamoto's Sen-Toku fleet was set in motion in 1942, and building of the vessels began in January 1943 under great secrecy. The shipyards at Kure (in the prefecture of Hiroshima) and Sasebo were tasked with the commission. Yamamoto's original plan required the construction of 18 supersubmarines, but his untimely demise on April 18th, 1943 (when his airplane was shot down over the Solomon Islands) caused the construction order to be scaled back to only nine vessels, and then even further down to just five due to shortages of wartime supplies.
In the end, only three Sen-Toku class vessels ----- the I-400 (completed in December 1944) and its two sister vessels (completed in early 1945) ----- were ever made. I-404 was 90% completed before it was damaged beyond repair by an Allied air raid, and the I-405 was stopped mid-construction due to waning interest. Both the I-404 and I-405 were subsequently scuttled and scrapped for their metal. The further thirteen Sen-Toku class aircraft-carrying supersubmarine vessels were cancelled before ever being built. Had the Sen-Toku class vessels been used to their full potential, they would have altered the course of the war.
The I-402 was completed in July 1945, just five weeks before Japan's surrender; it was converted to carry fuel to the Japanese fleet in the East Indies. Consequently, only the I-400 and the I-401 were commissioned and thereafter militarily deployed in early 1945, initially to attack the Panama Canal so as to disrupt Allied shipping and cripple Allied logistics.
As luck would have it, July 1945 saw both the I-400 and I-401 receive a change in their orders, with a revised combat mission against American forces massing near the Ulithi atoll in the Pacific. Both I-400 and I-401 were scheduled to launch their aircraft on 17th August in a coordinated effort with the Kaiten attack. But by then Hiroshima and Nagasaki were bombed, prompting Japan to surrender on 15th August 1945. Because the war ended, all three existing Sen-Toku vessels never completed their respective missions.
On 16th August 1945 the Sen-Tokus were ordered back to Kure. Japanese Captain Tatsunosuke Ariizumi, who had planned and rehearsed the original Panama attack before receiving new orders to Ulithi, refused to believe that imperial Japan had surrendered; he thought the announcement to be an Allied ruse. Ten days later, on 26th August, the I-400 received orders to hoist the flag of surrender and disarm. The I-400's three Seiran aircraft were run out of the sub's hangar and catapulted, unmanned and without ordnance, into the ocean. The Seiran floatplanes' bombs were dropped via derrick into the ocean floor, and the sub's torpedoes were disarmed and also fired out. All of the I-400's logs, charts, codes, and secret documents were destroyed.
On 27th August, the I-400's flag of surrender was spotted by Allied Task Force 38's aircraft, and the American destroyers USS Blue (DD 744) and Mansfield (DD 728) accepted the official surrender of the I-400 just 500 miles NE of Tokyo, Japan. Captain Ariizumi performed ritual seppuku, in line with Japanese bushido honor codes. His body was ceremonially wrapped in the ensign (flag) of the Imperial Japanese Navy and buried at sea. The next day the I-401 was located by the USS Segundo (SS 398). I-402 suffered an ignominious end as well, being scuttled by the US Navy in 1946 near the island of Goto Retto following US Navy inspections, whereas both the I-400 and I-401 were eventually brought back to Pearl Harbor for further study.
Under the terms of the treaty that ended the war, the Soviets demanded first-person evaluation access to the subs in 1946 because the Sen-Tokus were the largest submarines of the time (until the 1959 arrival of the ballistic missile submarine USS George Washington SSBN-598), and because each Sen-Toku vessel had sonar-dampening technology. The rising Cold War tensions between the US and the Soviet Union prompted the US Navy in 1946 to hastily sink the subs on May 31 and June 2nd off the Oahu coast with claims of no official knowledge of the precise location of the scuttled vessels, all in an effort to keep the innovative technology out of Soviet hands.
Then in 2005 the wreck of sister vessel I-401 was found in Hawaiian waters, but the whereabouts of the I-400 continued to remain a mystery. That is, until earlier this year, August 2013, when a Pisces research submersible from the Hawaii Undersea Research Laboratory identified the I-400 from its unique features ----- stern running lights, deck crane, aircraft launch ramp, and torpedo tube configuration. Official announcement of the find was only made this month, December 2013, because of necessary coordination with the National Oceanic and Atmospheric Administration (NOAA) and the Japanese government.
A statement from James Delgado, director of NOAA's Maritime Heritage Program in Washington DC, regarding the Sen-Toku class technological marvel reads as follows: "The innovation of air strike capability from long-range submarines represented a tactical change in submarine doctrine...The large I-400, with its extended range and ability to launch three M6A1 Seiran aircraft, was clearly an important step in the evolution of submarine design."
Delgado continued to say: "Following World War II, submarine experimentation and design changes would continue in this direction, eventually leading to ballistic missile launching capabilities for US submarines at the advent of the nuclear age."