Full Episode
Japanese SuperSub

Spring, 1946. Ten months after the end of World War II, an explosion rocks the Pacific off the coast of Hawaii. America has just destroyed one of Japan’s most advanced weapons systems – the I-401 aircraft carrier submarine. But this was no belated attack against the defeated Japanese. Bound by an agreement to share any discoveries with the Soviets but feeling the pressure of the looming cold war, it was a calculated decision to keep the technology out of Soviet hands.

The Japanese had built their sub to take the war to U.S. shores. Merging the stealth and tactical advantages of sea and sky, the revolutionary submarine carried three specially-designed Seiran attack bombers, which could be launched from the deck of the sub within seven minutes of it reaching the surface.

With missions to attack U.S. cities and blow up the Panama Canal, the aircraft carrier submarine had the potential to change the course of the war in the Pacific.  But fortunately for America, it’s secret weapon – the atom bomb, was put into action first. Japanese SuperSub investigates Japan’s efforts to take submarine technology where it had never gone before, and reveals how close the Japanese came to using the sub for an attack on the U.S.

Produced by Windfall Films and Spy Pond Productions for THIRTEEN in association with National Geographic Channel and WNET.ORG.


Producer/Director: ERIC STANGE


Co-Producer: Jackie Mow

Executive Producer, Windfall Films: DAVID DUGAN

Executive in Charge: William R. Grant

Executive Producer: Jared Lipworth

© Copyright 2010 WNET.ORG Properties L.L.C., Windfall Films Ltd. and Spy Pond Productions.

Transcript Print

Secrets of the Dead: Japanese SuperSub

Program Transcript:

NARRATOR: Spring, 1946.  Ten months after the end of World War II, an explosion rocked the Pacific off the coast of Hawaii. America had just destroyed one of Japan’s most advanced weapon systems.

CARLO CARLUCCI: I was a lookout. When I got to look through the periscope.  It was a monster, believe me.

NARRATOR: Rather than a belated attack against the defeated Japanese, the sinking of this top secret submarine was a preemptive measure for the looming Cold War—A brazen decision by the Americans to keep the sub out of Soviet hands. The plan worked, and the weapon lay undisturbed at the bottom of the ocean for six decades, until a team of underwater explorers from the University of Hawaii located its remains. The discovery sparked a new examination of the long-forgotten supersub.

ERIC GROVE: It can go around Cape Horn and then go around the Cape of Good Hope.

CARL BOYD: Thirty seven thousand miles.

ERIC GROVE: That’s right, this gave you enormous strategic options.  You could basically attack anywhere in the world.  This was a global weapon system.

NARRATOR: A system that defied conventional design, and married the tactical advantages of sea and sky. As America scrambled to build nuclear bombs, and Germany experimented with powerful rockets, Japan hoped its secret weapon would change the course of the war.


NARRATOR: 1941. Six months before the attack on Pearl Harbor, the Japanese Imperial Navy dominated the Pacific.

ERIC GROVE: The Japanese strike in December, 1941 because they see it as a window of opportunity. Japan temporarily has the most powerful Navy in the Pacific, let’s cash that in.

NARRATOR: The Japanese plan was simple.  Hit hard, and knock the Americans off balance.  They believed this would force an American retreat—leaving Japan as the sole superpower in the region. The architect of Japan’s Pacific strategy was Harvard-educated Admiral, Isoroku Yamamoto. Yamamoto had mixed feelings about the campaign.

OSAMU TAGAYA: In terms of his own personal opinions, he was very much against a war with the United States, which he felt was essentially unwinnable.  But at the same time, in his capacity as commander-in-chief of the combined fleet, he felt he had to come up with some kind of strategy to make a Japanese war on the United States a viable proposition.

ERIC GROVE: But nobody realizes more than he how wrong it can all go. And his gambler’s instinct tells him that it will have to be very long odds.

OSAMU TAGAYA: He wants some capability of, say, taking the war to American home waters, really shock the American populace by some bold gesture.

NARRATOR: His gesture:  The bombing of Pearl Harbor. The attack sank five battleships, three destroyers, and several small ships. But the Japanese mistimed their mission, because on December 7th, all three American aircraft carriers were at sea—and out of harms way. Japan also miscalculated America’s will to fight back. One day after Pearl Harbor, the U.S. declared war against Japan. The fight was on.  Both nations scrambled for advantage. The U.S. cranked up its industry and relied on its manpower—aiming to overwhelm the Japanese with superior numbers. But for Japan, the challenge was different.  Knowing he would soon be outmanned and outgunned, Admiral Yamamoto needed more surgical strikes. In the months following Pearl Harbor, he strategized about how to bring the war across the Pacific to America—54 hundred miles away. He looked to the success of the German U-boats—the deadly Nazi subs that were preying on ships in the Atlantic. From February to May, 1942, the U-boats sank 348 vessels, preventing millions of tons of American supplies from reaching Europe. Several of the ships were torpedoed within sight of New York City and Boston. If German subs could terrorize the U.S. East Coast, could Japanese subs do the same in the West? To find out, Yamamoto ordered a series of test missions. He sent a sub to fire shells at a refinery in California.  It didn’t cause much damage, but did trigger fears of a Japanese invasion.

ARCHIVAL FOOTAGE AUDIO: Along the Pacific, the hunt is on for the Jap submarine that brought the war to U.S. soil. Lying offshore, the marauder opened up against oil refineries near the beach at Santa Barbara. A piece of shell made in Tokyo: it did little harm, but it was the first fired in this war against our own shores.

NARRATOR: The following day, coastal artillery brigades fired at what they thought were enemy planes. The guns were shooting at shadows—no sightings were actually confirmed. But the panicked American reaction did signify to Yamamoto that his instincts were correct.  If he could strike the Americans at home, he might make them think twice about an all-out war with Japan. To instill such fear, he would need more firepower than a few small subs could deliver. An aircraft carrier and fleet of bombers would be ideal, but with the U.S. on high alert, no carrier could sneak up on the West Coast. Yamamoto needed something unexpected.  His solution: A superweapon that would unite the firepower of an aircraft carrier with the stealth of a submarine. If it could be built, his machine would rewrite the rules of warfare. The concept of putting a plane on a sub wasn’t new. But conventional submarines of the day were only capable of carrying one small plane for reconnaissance and targeting. Yamamoto wanted sub-launched airplanes that could be used as an offensive weapon, not just a scout. He ordered another test mission. This time, a small, submarine-launched aircraft dropped fire bombs over Oregon in the hopes of sparking a forest fire. The fire failed to catch, but like the Santa Barbara mission, it confirmed that a submarine could slip past U.S. coastal defenses and strike unsuspecting civilian populations.

STEPHEN McFARLAND: To be able to reach your destination and launch an air attack undiscovered, undetected, would give the Japanese an advantage that no other Navy, that no other Air Force had during the war.

NARRATOR: Yamamoto ordered his engineers to design a fleet of underwater aircraft carriers capable of sailing undetected across the Pacific…launching squadrons of high-tech bombers to attack West Coast cities…then disappearing without a trace. The admiral even expected his subs to be able to reach the East Coast. He hoped to terrorize America with attacks on New York City, and possibly even Washington DC.

ERIC GROVE: So you’ve got a whole potential here for attacks in the coastal area where of course a large proportion of the American population is concentrated.

CARL BOYD: That’s right.

OSAMU TAGAYA: And New York City for example. It’s the dense concentration of population.

ERIC GROVE: And if you’re attacking with a few tens of aircraft as in the original plans, you could perhaps go for important communications and indeed iconic targets.

NARRATOR: The Admiral names the sub the I-400, and declared it top secret. But still to be determined was whether Japanese engineers could bring Yamamoto’s ambitious vision to life. Their window for success was a small one.  In America, the U.S. had accelerated its own work on a super-weapon that could change the course of the war. The effort was code-named The Manhattan Project. It was headed up by Robert Oppenheimer, a professor of physics at U.C. Berkeley. Oppenheimer gathered the nation’s top engineers and physicists—challenging them to overcome the enormous technical challenges of splitting the atom and creating an atomic bomb. The Americans worked at a furious pace, fearing that the Germans and Japanese were moving forward with nuclear bombs of their own. Meanwhile, Japan’s super-submarine was running into obstacles.  In Tokyo, naval architects were struggling to find a design that could fulfill all of Yamamoto’s needs.

ERIC GROVE: You’re going to need a very large and impressive submarine indeed.  You’re going to have to go into new territory, in fact, as far as submarine design is concerned.

NARRATOR: The standard submarine at that time was shaped like a cigar, with a cylindrical hull up to 300 feet long. But no one knew whether that conventional design would be able to support a heavy hanger and three attack airplanes.

ERIC GROVE: You’re going to need a very large hangar beginning there, moving past the super structure and ending up about there. What do you do with the navigational area here—you have to move it to the side of the ship and on top as well so that’s a lot bigger. These contain very large aircraft, that’s not long enough so you need a much, much longer flying-off deck and therefore you need a much, much bigger submarine. This is a transformation in the whole concept of the submarine.

HAROLD VINCENT: When we assemble these pieces, we’ll end up with a scale-model submarine that’s about the same as a full-size submarine, which is ten times longer than it is wide.

NARRATOR: Dr. Harold Vincent is an ocean engineer at the University of Rhode Island.  He has his doubts about the practicality of the design.

HAROLD VINCENT: This tube represents the watertight hangar, the three aircraft, and the bombs that the aircraft carry. So we’ll seal off the end of the hangar here, so let’s see what happens when we put it in.

STUDENT: So that tilts right over with that on top.

HAROLD VINCENT: Capsizes right over and sinks. So therefore they had to come up with some other method to be able to put a heavy hangar with all that aircraft up high out of the water.

NARRATOR: Adding the hanger to a conventional sub clearly wasn’t the answer.  The next, seemingly obvious approach would have been to scale up the size of the central cylinder to lower the center of gravity.

CARL BOYD: Well, a large single cylinder would be extremely heavy in weight, would have to have very thick walls in order to withstand sea pressure down to three hundred or three hundred and thirty feet in order to hold this above the surface part of the submarine totally stable was simply impossible having this traditional submarine design.

HAROLD VINCENT:  Tape up the other end, and then we can tape these…

NARRATOR: The Japanese came up with an innovative alternative.

HAROLD VINCENT: Harold: So now we’ll try this twin hull design. Now, it’s pretty stable!

Student:  All right, so it rights itself now instead of just tipping right over.

NARRATOR: A twin-cylinder hull gave the giant sub the necessary width to carry the extra weight.

ERIC GROVE: So you have these two cylinders which are broader and stronger.

CARL BOYD: The two cylinders at mid-ship of course lent to greater stability.

ERIC GROVE: Absolutely.  Well you can see it’s much flatter and more stable clearly.

NARRATOR: With the largest design problem solved, construction plans were drawn up for the new, monster sub. The I-400 went into production in January of 1943. Yamamoto needed it fast. The Japanese were losing ground in the Pacific. The previous June, Yamamoto had ordered a surprise attack on the American fleet at Midway Island, hoping to sink the aircraft carriers he had missed at Pearl Harbor. But the Americans had cracked the Japanese naval code, saw the attack coming, and were able to set an ambush of their own. In three days of bitter fighting, American bombers sank four of Yamamoto’s aircraft carriers.  It was a devastating defeat for Japan. Their carrier fleet was decimated and thousands of soldiers were dead. The I-400s were now even more vital. But with serious shortages of steel and manpower in Japan, Yamamoto could only commission eighteen of the giant submarines. With work on the first I-400 underway, the Japanese Navy began development on the secret bomber that would be carried in the watertight hanger on the deck of the sub. The plane was called the Seiran, meaning “mist on a fair day.”

ATSUSHI ASAMURA: Like the name indicates, the plane would appear suddenly like a mist-- carried by a submarine that would surface once it comes near a target, moving like a ninja. That is how it came to be named “Seiran.” This is a very poetic name.

NARRATOR: Lieutenant Atsushi Asamura is the only living member of the elite Seiran squad. At an annual Shinto ceremony, he honors his lost comrades.  He still recalls the excitement of being part of the top-secret project.

ATSUSHI ASAMURA: We were very surprised when we saw the Seiran for the first time. I thought, “Is this really a plane that can be loaded onto a submarine?”

I believe not too many people knew about Seiran even in the Japanese Navy. The many tests and repeated experiments were done in top secret.

NARRATOR: With a maximum speed of more than 200 miles per hour and the capacity to carry a seventeen hundred pound bomb, the newly-designed Seiran would be an intimidating warplane. But the Japanese aircraft designers first had some technical puzzles to work though. Although the I-400 was wider than any other submarine of its day, the airplane hangar was only eleven feet in diameter.

OSAMU TAGAYA: Here is the hangar on the I-400 into which this aircraft has to fit.  And here we have a head-on view of the Seiran.

ERIC GROVE: In the same scale.

OSAMU TAGAYA: In the same scale.  You know, the fuselage will fit in there, but gosh, you’ve got the wings, you’ve got the tail plate and it just doesn’t fit.

NARRATOR: To accommodate the tight quarters, the Japanese designed wings much like those on the Grumman Hellcat—the most potent aircraft-carrier-based fighter in the U.S. arsenal. To minimize the Hellcat’s profile for storage below-decks, the wings rotated ninety degrees and folded back flat against the fuselage. The Seiran went even further. It had a tail fin that folded down to reduce height.

OSAMU TAGAYA: Osamu: When you do all of that, it fits, very neatly.

NARRATOR: But there was still one major problem the Japanese designers had to solve. Before the Seirans could be launched, their engines had to be warmed up—a process that took up to twenty minutes. Starting the engines in the hangar with the vessel submerged would have exposed the crew to deadly carbon monoxide fumes. But warming the engines on the surface meant exposing the sub to radar and air attacks. Once again, the engineers needed an innovative solution. At a small airport in Connecticut, former air force gunner Craig McBurney knows all about the problems of warming an engine before takeoff. This is his pet project, a rare, 28-cylinder Corsair engine.  It’s larger than the Seiran’s engine, but shares similar characteristics.

CRAIG MCBURNEY:Okay, I’m going to clear prop.

CREW:  Clear prop.

NARRATOR: As with many World War II-era aircraft, starting the engine cold is a haphazard and messy affair.

CREW: Right about now is when we start using bad language.

NARRATOR: The problem is the viscosity of the cold engine oil.

craig McBurnEy: You can see how thick the oil is because it hasn’t been heated up and you know how critical the tolerances are inside the aircraft engine, how tight they are, and how small the passages are, so it would really make a significant difference trying to pump that heavy, thick oil through an aircraft engine.

NARRATOR: Warming the oil makes a noticeable difference.

craig McBurnEy: We heat it up to about 60 or 70 degrees Celsius, which is 140, 160 degrees Fahrenheit. It’s the same oil that’s just been heated up with the preheater.  Right through it. Almost, like pouring water. What a difference. And you can imagine how critical that would be to have the engine heat up that much faster, especially in a combat situation.

NARRATOR: McBurney uses an external tank to preheat the oil.

CRAIG MCBURNEY: Inside this container, we have the main engine oil tank here for feeding the engine oil. Then we’ve also installed our own pre-oil tank with a heater on it. And with the pre-heater on it we’re able to raise the temperature quite a bit – we can get the temperature up to about 120 degrees. So we need to open this valve. And we’ll go ahead and open the engine oil tank valve as well.

NARRATOR: The warm oil can be pumped directly into the cold engine.

CREW: Clear prop. Go ahead and engage the starter.

NARRATOR: The results are easy to see.

CREW: That’s incredible; temperature is already at the takeoff temperature.

CRAIG MCBURNEY: Amazing difference, can you imagine in a combat situation?

NARRATOR: McBurney’s warming method was the same one Japanese engineers turned to for the Seirans.  It’s actually borrowed from a German design.With the engine oil pre-warmed, the plane could be rolled out of the hangar onto the launch ramp.

The engine started…

The wings, tail and horizontal stabilizers unfolded and locked into position…

The floats attached….

And the Seiran launched into the air…

ATSUSHI ASAMURA: We would train very hard trying to shorten the time it takes for the launch, even by a second.  The first plane launches, then the second, and then the third.  The goal is to launch all three planes as fast as we can.

NARRATOR: To get the Seirans back on board, the engineers designed a hydraulic crane to pluck the airplanes from the sea and hoist them onto the deck. With all the design problems solved, it looked as if the Japanese would be the first to get their super weapon into the war. But then, in April of 1943, the I-400 program—and the Japanese Navy—suffered a devastating loss. American code-breakers discovered that Admiral Yamamoto was planning an inspection tour of the Solomon Islands. U.S. fighters intercepted and shot down his plane, killing the man behind the Pearl Harbor attack.

STEPHEN BUDIANSKY: It was such a crushing blow to the Japanese Navy that it was a full month before they even announced Yamamoto’s death to the Navy, much less the Japanese public. Without the backing of the powerful Admiral, the I-400 program quickly slipped on Japan’s priority list. Before any subs were completed, the order was slashed from 18 to 9. It would take another year and a half before the first of Yamamoto’s I-400s made it out to sea. In December of 1944, the first I-400 was finally commissioned. A few months later a second sub, the I-401, was ready to sail. The supersub carried three dive-bombing Seirans in a 65-foot-long hangar.  An 85-foot ramp and steam-powered catapult launched the planes into action, even in rolling seas. The I-400 wasn’t just the longest sub in the ocean at 400 feet; it was also the most heavily armed. On the aft deck sat a giant 140-millimeter gun—one of the largest ever mounted on a submarine. Four anti-aircraft guns defended against aerial attacks. And, the sub also boasted 8 torpedo tubes in the bow. The man appointed to lead the I-400 program was Tasunosuke Ariizumi.  Ariizumi had been in charge of the midget submarine attack on Pearl Harbor. The I-400’s crews were picked from the navy’s elite—and were very well-treated.

TSUGIO YATA: The corridor was filled with cans of food and other food staples. You couldn’t even see the floor. I was never unhappy with the food - for example we even had expensive food like cow tongue and lots and lots of fancy stuff. I remember it well.

NARRATOR: Morale among the crews was strong, but Japanese High Command was not as jubilant.  In the three years since the I-400 project began, the state of play in the Pacific had changed.  America now dominated the region, and the original mission of the subs—to bomb U.S. cities and instill fear of a Japanese invasion—was outdated. They just didn’t have enough firepower. With each sub carrying three planes and each plane carrying only one bomb per flight, they could do little real damage. By comparison, Nazi bombers dropped an average of 330 bombs per night for 57 nights during the London Blitz. And still, the British didn’t surrender. With a conventional bombing raid on U.S. cities out of the question, the Japanese needed another mission for the I-400s. They considered all options.

NORMAN POLMAR: As the Japanese were becoming frantic for any means of inflicting more pain on the United States, in an effort again to slow our advance or get us to negotiate a peace, the Japanese considered using germs against American cities. This was discussed and the obvious means of delivering these would have been by submarine-launched aircraft from the I-400s, from other submarines.

NARRATOR: In the spring of 1945, Vice Admiral Jisaburo Ozawa proposed a top secret, controversial plan.  He suggested using the Seirans to unleash biological weapons on the U.S. West Coast. Such an attack could kill thousands, and create panic across America.

NARRATOR: Japan’s biological weapons program was not new.  Under the command of a military doctor named Shiro Ishii, a secret team had been experimenting on the Chinese since the 1930’s. Plague-infested fleas bred in their lab has been unleashed into Chinese villages. They had infected prisoners with anthrax and cholera. And they had injected men and women with venereal diseases. As many as 200,000 Chinese died as a result of these horrific experiments.

ERIC GROVE: The Japanese Army was doing a lot of work on this in Manchuria, in pretty awful circumstances actually, and certainly the Japanese Navy seems to have been interested in the use of Anthrax in bombs.

CARL BOYD: And the most radical elements within the Army or the Navy would advocate such a, well I would say insane plan.

NARRATOR: There was little doubt that biological weapons dropped on an American city could cause enormous casualties. And sow panic among the American population far more effectively than conventional bombs. But a month later, cooler heads prevailed. General Yoshijiro Umezu of the Imperial Army, canceled the operation and declared: "Germ warfare against the United States would escalate to war against all humanity." With a biological attack off the table, the Japanese High Command found a new mission for the I-400s: An attack on a key strategic target.

CARL BOYD: The Panama Canal was absolutely crucial to the transferring of the Atlantic Naval forces to the Pacific Ocean.

ERIC GROVE: Which is why the Japanese wanted to attack it.


CARL BOYD: Well, of course. And we understood that. Any logical military scenario would want to attack the Panama Canal.

NARRATOR: If Japan could take out the Canal, it would bring ship traffic to a halt and force the Allies to use the much longer route around Cape Horn. Japanese military planners and intelligence experts worked together to map out the attack.

ERIC GROVE: The plan went something like this.  This flotilla of submarines, four submarines carrying ten aircraft, would come keeping a due distance, having come a long way across the Pacific.

OSAMU TAGAYA: Coming a long way across the Pacific.

ERIC GROVE: 8000 miles or so…until they come to their launch point off Ecuador, roughly about there. Then they surface, and they launch their planes, the six aircraft, as quickly as they can. And so as to surprise the enemy completely, they go off in completely a different direction. They fly across Colombia, then they turn sharply, over here, and then suddenly, and they perhaps dive to low level, they come down here towards the canal and hit it on its northern end in the locks here.

NARRATOR: The mission would be extremely hazardous for the I-400 fleet. Before striking the Canal, the subs would have to navigate through waters swimming with Americans.

STEPHEN MCFARLAND: By the summer of 1945, essentially the Pacific was an American lake.  And so that meant it would be relatively difficult to get any kind of vessel safely into the area of the Panama Canal.

NARRATOR: But the Japanese did have one ace up their sleeves—a secret technology that the I-400’s designers had borrowed from the Germans. A technology that could help them elude enemy sonar.Sonar works by bouncing sound waves off hard objects like ships. By measuring the time it takes for sound waves to travel to the target and back, the system can calculate the location of the target ship. But the I-400s, like some German U-boats, were equipped with a new stealth shell, designed to absorb sound waves instead of reflecting them. Called an anechoic coating, it was made up of rubber and asphalt tiles that dampened the sonar fingerprint of the suba as they slid through the water.

ERIC GROVE: These anechoic coatings did work remarkably well and their details are still highly classified.

NARRATOR: For the I-400s, getting across the Pacific undetected was only step one.  Their targets—the Gatun Locks on the Panama Canal—were heavily guarded by anti-aircraft guns. The Seirans could come in above the guns’ range, but that would make hitting the targets almost impossible.

STEPHEN McFarland: Those locks, the walls, were incredible structures of reinforced concrete that at the base were 50 to 60 feet thick, at the top maybe 8, 10, 12 feet thick. The gates themselves maybe 90 feet wide, they were six, seven, eight feet of steel thick.  But from 13,000 feet, they would have looked like just a hair from that kind of altitude.

NARRATOR: The challenge of high-altitude bombing is easy to see.  When a B-29 outfitted with a World War Two-era bombsight drops a half-dozen water bombs on a stationary target in the California desert…not a single bomb hits home.  The closest lands more than 500 feet away.

STEPHEN MCFARLAND: For a target like the Gatun Locks, anything less than almost a direct hit within a foot or two or three of the target would be a waste of energy. Given the relatively primitive bombsights that the Japanese were using, their chances of being able to hit a target like that were pretty slim.

NARRATOR: And with only six planes carrying one bomb each, the Japanese knew there was no room for error. Keeping with Japanese military tradition, a decision was made to turn the attack into a one-way trip.  The pilots were ordered to go Kamikaze.

OSAMU TAGAYA: By that point in time, just about all Japanese air missions were being flown as kamikaze missions, one-way missions, and the plan was to do the same here.

ATSUSHI ASAMURA: Given our lack of resources at the time, in Japan, we had no choice but to go Kamikaze. I know it is controversial, some people are against it. But I think we had no choice at the time. We knew we probably couldn’t hit the target even with multiple bombs.  So going kamikaze was our only chance.

NARRATOR: The I-400s’ crews prepared for the Panama Canal mission, but the window of opportunity for the strike was closing fast.  By the Spring of 1945, America was already deciding on potential targets for the atomic bomb. Then, the war arrived on Japanese soil when Allied forces invaded the island of Okinawa. 82 days of brutal fighting left more than 100,000 Japanese troops dead, and thousands of aircraft and combat ships destroyed. Vice Admiral Ozawa realized that a raid on the Panama Canal would be too little, too late. The majority of American forces were already in the Pacific.  So Ozawa changed the I-400s’ mission yet again. The new target was Ulithi Atoll—a staging area for the massive U.S. fleet preparing to invade Japan.

ATSUSHI ASAMURA: Ulithi was the likely base for the units that attacked Okinawa and perhaps also those that were to be sent to the final confrontation on the mainland; supplies were probably sent from there as well. In the photo I saw several U.S. aircraft carriers. Headquarters ordered us to attack as many of them as possible.

NARRATOR: Three and a half years after the I-400s were conceived, the two giant subs were finally ready to engage.  They traveled separately, with orders to rendezvous off Ulithi, where they would be joined by two smaller subs. Once they reached their target, six Seirans would be launched in a kamikaze attack. To deepen the surprise, Commander Ariizumi ordered the crew to disguise the bombers with U.S. markings—a clear violation of international law.

OSAMU TAGAYA: Pilots themselves objected to having the planes painted in American colors.  They felt it was dishonorable and they were ready to die, they knew they weren’t coming back.

NARRATOR: What they didn’t know, was that their mission was once again short on time. Four days before the I-400s set sail, America had successfully exploded the world’s first atomic bomb. A second bomb was already on its way to Japan. To make matters worse, Ariizumi’s mission was plagued by problems. On the way to Ulithi, one of the smaller subs was spotted and sunk by a U.S. warship. All 140 crewmen perished. Then, in an effort to avoid enemy ships, Ariizumi decided to change his route, and the agreed-upon meeting point. But the message never got through to the other ships, and they missed the rendezvous. As the I-400 and I-401 scrambled to regroup and renew the attack, shocking news came over the radio. The American super-weapon had blown the war wide open. On August 6th, 1945, an American atom bomb annihilated the city of Hiroshima, killing as many as 80,000 people. On August 9th, a second bomb decimated Nagasaki.  40,000 died and the city lay in ruins. Six days later, Japan surrendered to the United States.


NARRATOR: In his radio address to the nation, the Emperor never mentioned defeat or surrender. Instead, he told his people: “we have to endure the unendurable and suffer the insufferable.”

ATSUSHI ASAMURA: The next day on the 16th, we received a telegram ordering us to cease fire. Receiving this unthinkable telegram, we gradually grasped the reality. From then on, the real drama began on board.  There was no concept of surrender and remaining in the Japanese Imperial Navy at the time.

NARRATOR: The Captains of the two I-400s reversed course, and raced back toward Japan. Neither wanted to give up his prized ship, nor suffer the humiliation of being captured in enemy waters. Worried about retribution, Ariizumi ordered the crews of the two subs to dump their U.S.-marked Seirans into the ocean.  They did so, destroying the evidence just in time. On August 28th, two U.S. destroyers discovered the I-400. The Captain surrendered peacefully.  20-year-old Harry Arvidson was one of the first Americans to board the giant sub.

HARRY ARVIDSON: As we approached the submarine and saw what it was, I thought to myself ‘Man that’s the biggest thing I ever seen.’

NARRATOR: The next day—August 29th—the American submarine USS Segundo located the second giant sub.  The I-401 was carrying Fleet Commander Ariizumi—a man who didn’t know the meaning of the word ‘surrender.’ Carlo Carlucci was a quartermaster on the American sub.

CARLO CARLUCCI: It had to be three or four o’clock in the morning, before daybreak.  That was when they picked it up on the radar. They didn’t know what it was until they got fairly close.

NARRATOR: Tsugio Yata was a young sailor on the I-401.

TSUGIO YATA: We didn’t want to give up this magnificent ship to the U.S.  We thought it’s a shame for it to be captured.

NARRATOR: Though the war was over, surrender did not come easy for the prideful Japanese sailors.

CARLO CARLUCCI: Things were hot and cold.  You never know yes or no, were they going to shoot, not shoot.

TSUGIO YATA: We were heading back to Japan but we talked about how to prevent the Americans from taking the sub so we talked about sinking it near the coast.

NARRATOR: But before the Japanese could carry out their plan, the sub’s engines temporarily failed. The Segundo signaled for the massive sub to surrender.  Ariizumi refused. Several tense hours passed with the Segundo’s weapons trained on

the I-401. Muneo Bando remembers the capture.

MUNEO BANDO: They pointed their cannons at us and ordered to have one officer sent to their ship.  We thought, “What do you mean, ‘Send someone.’ You send someone to come get us.” We insisted on displaying our pride and dealing with them man to man.

NARRATOR: Because Bando spoke some English, he was picked to negotiate with the Americans.

MUNEO BANDO: I said, “If you force us to surrender, we will commit suicide.”  They said to me, “Harakiri, no good.”  They understand the term harakiri.

NARRATOR: Tokyo ordered the I-401 to surrender to the Segundo.  Eventually, Commander Ariizumi bowed to the inevitable.  He had the black flag—the international naval signal of surrender—raised on his sub. For the proud Commander, the humiliation was too much to bear.

TSUGIO YATA: I went to get a cup of coffee and had just sat down, I heard a loud bang. That was the suicide. The Captain and I rushed inside, and we saw he’d taken his own life.  I heard the Captain say “He finally did it!”

NARRATOR: Nearly 200 Japanese servicemen were taken prisoner that day—sailors, pilots, and support crew. The American submariners seized control of the giant sub, and quickly realized it was unlike any vessel they had ever seen. For machinist Paul Wittmer, the double-hull design was a particular surprise.

PAUL WITTMER: Lo and behold we get to the engine room and there’s a doorway to the neighbor’s room next door.  What the heck is going on here?  We take a peek in there and there’s another set of engines in there.  The port and starboard engine rooms.  We find out that there were two hulls bolted together—two submarines along side each other.

NARRATOR: The Japanese crew remained on board, but with six Americans now in command.

CARLO CARLUCCI: I had a 45, we all had 45s and I had an extra clip of bullets in my jacket pocket.  If I had to use it, I would have used it.

PAUL WITTMER: It was very tense, no one trusted anybody.  We didn’t know what to expect. We could have been overtaken, lickety-split, because we were way out numbered.  And if they decided to dive the boat we couldn’t have gotten out.

NARRATOR: The Americans chained the hatches open to prevent the Japanese from diving.  But as time passed, tensions slowly eased between the former enemies. They now had a common goal: Getting the sub safely back to Tokyo.

PAUL WITTMER: In the engine room, we have to learn how to communicate with the Japanese. They had to teach us and we had to teach them. We needed to know some of the words to describe an engine. Basic elementary, grade school type communication, and trying to strike up a rapport and learning their symbols and their words for what’s the name of this, what’s the name of that.

NARRATOR: Both subs made it back to Tokyo, where the prisoners were released.  For the Americans, the next step was to bring the unusual ships home for further study.  In November of 1945, they departed Japan for Hawaii. They arrived in Pearl Harbor just after New Year’s Day, 1946. Navy engineers immediately began inspecting and recording every detail of the supersubs’ design. But by the spring of 1946, a new, post-war reality had taken hold, and the I-400s were once again shrouded in secrecy.  This time it was the United States hiding them from the Soviet Union.

ERIC GROVE: Large aircraft-carrying submarines intended for strategic attack on their enemies are exactly the things you don’t want the Russians to have.

CARL BOYD: And this is now the Cold War.

ERIC GROVE: The beginning…

OSAMU TAGAYA: The beginning of the Cold War.

NARRATOR: Worried that the Russians would demand to inspect the subs, the U.S. Navy made a hasty decision. On May 31st, 1946, they sunk the I-400 off the coast of Pearl Harbor. Two days later the I-401 joined it at the bottom of the sea. Two powerful weapons that never made it into battle. That never had the chance to truly prove their worth.

OSAMU TAGAYA: If the question is would the I-400 operation decisively change the course of the Second World War?  Ultimately, no I don’t think so.

CARL BOYD: It would have made things worse in 1942, but not in 1945.

ERIC GROVE: And Japan surrenders because of the overwhelming material superiority of the United States. The United States has got the willingness and the desire to deploy that overwhelming power.

NARRATOR: Ultimately, the I-400 arrived too late in the war to make a difference.  But while its timing was flawed, its technology was ahead of its time. The innovative design became a model for future Cold War submarines, and changed military thinking about how they could be deployed. In the 1950’s, a new type of U.S. submarine—the Regulus Class—began patrolling the seas. It bore a striking resemblance to Japan’s World War II supersub, though it launched missiles, not airplanes from its deck-mounted hanger.

NORMAN POLMAR: I don’t believe anyone previously had looked at submarines as a means of attacking an enemy’s cities. And this idea we see today in the primary nuclear weapons of the United States, France, Britain and even Russia being submarine-launched missiles to attack with nuclear warheads, an enemy’s cities.

NARRATOR: Though we hope that their lethal force is never needed, each one of these powerful, stealthy submarines is a living testament to one of the greatest weapons that never did battle.