The I-400 Class: Japan's Underwater Aircraft Carrier

Every so often when I research World War II history for material for this channel, something jumps out at me and it's so compelling that I get completely pulled down a rabbit hole to find out more about it. And the story of the I-400 Super Sub is exactly one of those things, an idea so bold and so crazy that it should never really have worked. But unlike the Nazis who seemed to do nothing but spend the last years of the war inventing needless or completely implausible plans, the Japanese took a very bold idea and very quickly turned it into quite an effective weapon. Various nations played around with the idea of submarines that carried aircraft before with very very little success. Both the French and British had designed large submarines with pressurized aircraft hangars built into the hull and these efforts had largely ended in tragedy with the loss of the boats and their crews. The German and American navies had made some limited explorations into this field, but generally the conclusion found was that other than increasing the reconnaissance range of the submarine, there wasn't very much value in doing it. But during World War II, it was a concept that was seized upon by Isoroku Yamamoto who was the commander-in-chief of the Imperial Japanese Navy and the architect behind the raid on Pearl Harbor. In the aftermath of that raid, when the launched the Doolittle attack in 1942, there were limited Japanese reprisals on the US mainland. As stated, they were quite limited in scope and largely consisted of the use of deck guns and float planes from Japanese subs, as well as some weaponized air balloons with explosives attached to them. These attacks caused quite little damage and loss of life, but they did cause a lot of panic and concern to the American authorities and public and that was something that Yamamoto wanted to capitalize on. His vision was to commission a fleet of giant submarines, each of which was capable of launching multiple aircraft. These could then arrive undetected off the US coast, launch in a massed attack and cause widespread damage and destruction before then being abandoned in the sea near the submarines recovered and then leaving and avoiding capture. What the Japanese planners came up with was the I-400, an absolute beast of a boat, 120 foot long and displacing six and a half thousand tons when submerged. It would be the biggest submarine the world would see until 1965 with the introduction of the Benjamin Franklin class in the US Navy. To go with this brand new class of submarine, a whole new seaplane had to be designed to accompany it, the Aichi Seran. This was a two-seater monoplane, it had wings that folded at 90 degrees and float that could be bolted off and on again and was designed to carry up to a 2,000 pound torpedo or bomb load. Three of these planes could be folded away and stowed into a 100 foot hanger which was built into the deck of the I-400 with the sub's conning tower being offset slightly to accommodate it. The crews would train night and day until they reached a point where they were able to unpack and launch all three of the seaplanes within 45 minutes, the idea being that once the planes had completed their bombing run they would then ditch or land in the sea close to the sub and be recovered by a retractable crane built into the hull and they'd be pulled back on deck and bolted and stowed away again. For protection the I-400 bristled with an array of triple mounted 25 millimeter anti-aircraft cannon, she also had a 14 centimeter naval gun and eight torpedo tubes, basically a veritable floating fortress. The boats were also covered in a special type of paint which was designed to reflect the effects of radar and overlaid with cables that were demagnetized, the idea behind them being they could counter the effects of the magnetic mines being used by the enemy. Yamamoto's vision called for 18 of these ships which would then join up off the U.S. coast and carry out massed attacks. He was killed in April 1943 during a targeted attack by the U.S. Air Force acting on covert intelligence. With him gone his opponents inside the Imperial Japanese Navy began to argue about the large number of resources allocated to the project and the number of boats was gradually reduced until in the end only three were completed. When they were launched in January 1945 the three super subs were ordered to prepare for an attack on the Panama Canal. This operation was codenamed Clear Sky Storm and the plan would see all nine of the seaplanes launching a surprise attack on the Gatun Locks. The destruction of the locks in theory would then empty the Gatun Lake which would make the Panama Canal impossible for shipping to navigate for several months and severely hamper the U.S. war effort. But in June 1945 as Okinawa was about to fall to the U.S. forces the Japanese Navy elected to cancel this raid and instead use the super subs to launch an attack on the U.S. fleet which at that time was anchored in the Caroline Islands. This operation was designated Mountain Storm and was a night attack which would necessitate the pilots of the planes having to have a hormone injection which would improve their night vision. In reality nine planes with limited bomb loads could probably cause limited damage to the U.S. fleet even if they gained complete surprise but the best result that the Japanese would have been hoping for would have been psychological damage caused by a successful attack. Having painted their aircraft in American colors to confuse their enemy I-400 and I-401 set off in August 1945 to complete the mission but following a series of navigational errors they eventually failed to locate one another. They were still at sea when the order to surrender was transmitted by the Emperor. I-402 at that time was laid up in her home base at Kure being repaired as a result of bomber damage and she was later taken into custody by the U.S. Navy. On the 27th August the crew of the I-400 assembled on deck they jettisoned their code books launched off their torpedoes and then assembled their aircraft which they placed onto the catapult without the pilots and launched in an effort to avoid them being captured and inspected by the enemy. A short time later they were detected by scout planes from Task Force 38 and the sub was detained and boarded by sailors from two destroyers USS Blue and the USS Mansfield. The following day lookouts aboard the submarine USS Segundo detected a large unknown vessel in the distance which they began to head towards. This target immediately made off from them with the American crew believing that what they were following must be a surface vessel because of its sheer size. To their shock upon catching up with it they discovered it was the I-401 which initially refused to surrender to them. Whilst the Americans were sat nearby repeatedly signaling for the Japanese submarine to surrender there was an argument raging below its decks. It transpired that the ship's executive officer was wanted by the American authorities for war crimes. He'd previously commanded the submarine I-8 and there was testimony that he had allowed the survivors of ships he'd sunk up onto the deck of his sub where he and the crew had then killed them using samurai swords and tools and certain survivors had testified to this effect. When the captain of the sub refused to agree to his demands to scuttle the ship the executive officer then retired to his cabin and shot himself. The crew were able to quietly jettison his body off the side of the ship without the Americans noticing before they too were boarded and captured. In April 1946 the Americans were still conducting tests and research on the three super subs when orders came through to sink them. This was in line with a larger operation which was designed to deprive the Japanese of their fleet but in the case of the super subs the Russian authorities were demanding access to the technology and seeing the way things were going the Americans decided it would be better just to sink them. I-402 while still in Japan at the time it was taken up to a point off the Goto Islands, lashed to another submarine and then sunk as gunnery practice for a nearby boat. The I-400 and 401 were at Pearl Harbor at that time still undergoing inspections. They were taken to a point off the coast of Hawaii and then sunk using experimental torpedoes. Both wrecks were rediscovered in March 2005. They lie at a depth of about 800 meters and are still in quite remarkable condition. The technology and the techniques that were used to create these ships were far ahead of their time and if this project had come to Yamamoto's full vision then potentially if used correctly these weapons could have had a significant impact on the U.S. war effort. Several men would lose their lives not only in testing these experimental aircraft and ships but also in commanding them and whilst the concept is not one that has been taken on by Navy since, the technology used in them would eventually be incorporated into the very U.S. subs that were going to dwarf their Japanese predecessors.