In August of 1945, as the world celebrates
the end of the Second World War, out in the pacific the Americans make a puzzling discovery. The US Navy has intercepted a Japanese submarine
and it’s unlike anything they’ve seen before. Its scale is baffling. But not only is it the world’s largest submarine,
it's an entirely new kind of weapon. A submarine that can launch torpedo dive bombers. The American’s have just stumbled across
Japan’s secret underwater aircraft carriers, and soon they’ll uncover a sinister plan
that could’ve changed the course of the war. Japan’s surprise attack on Pearl Harbor
in December 1941 is an event that galvanizes a nation. Pulling a reluctant America into the Second
World War. “The unprovoked and dastardly attack by
Japan on Sunday...” A day after the devastating attack, the United
States declares war on Japan. And the nation quickly mobilizes, firing up
its enormous industrial might to crank out ships, tanks and aircraft at a rate that’ll
soon bury Japan’s military. For the Americans, Pearl Harbor was a senseless
and cowardly provocation. But for the Japanese, the attack was something
entirely different. A calculated gamble and a long shot attempt
at actually trying to avoid a full-scale war with the United States. Because as the Empire of Japan continued its
ruthless conquest in Asia, the Japanese were convinced that it was only a matter of time
before the U.S. intervened. And the architect behind the Pearl Harbor
attack, Japanese Admiral Yamamoto was aiming to knock out much of the U.S Pacific fleet
in one decisive blow. As a way to keep the United States out of
the Pacific for at least another six months and perhaps even forcing the Americans into
negotiating a truce. But Imperial Japan grossly underestimated
America's resolve. And in the aftermath, Yamamoto knew America's
military might would soon overwhelm Japan’s. Now faced with a war he cannot win, Yamamoto
devises another strategy. To make America reconsider a drawn out war
in the Pacific, He’ll bring the war directly to America’s Cities. But with the United States now on guard for
Japanese forces, Yamamato will need a truly stealthy weapon to reach the United States. A weapon the Americans would never suspect. The concept of launching aircraft from a submarine
originated before the Second World War. But these earlier attempts were experimental
trials usually involving a single lightweight reconnaissance plane. What Yamamoto had in mind was far more ambitious. A fleet of submarines that could carry multiple
attack aircraft and strike fear into the enemy by launching surprise attacks on cities, only
to submerge and disappear again In March of 1942, Japanese engineers were
handed the enormous task of designing Yamamoto's secret weapon. To start, Yamamoto's aircraft carrying subs
would need to be capable of launching full-size torpedo dive-bombers. And engineers would have to design a catapult
launching system. And a mechanism to recover the aircraft and
bring them back onboard. But making the bombers fit inside a submarine
would be an even bigger challenge. With a typical World War Two-era dive bomber
having a wingspan of about 12 meters, engineers would also need to design an entirely new
dive bomber. One that could be folded up to fit inside
the sub’s hanger. The submarines would also need to be able
to reach any part of America's coastline, thousands of kilometers away. And then return all the way back to Japan
without refueling or resupplying. And it meant carrying nearly two million liters
of diesel fuel and enough supplies to support a crew out at sea for months. Japan’s secret underwater aircraft carriers
would be designated as the I-400. And they’d be enormous. Nearly twice the length of a typical German
U-boat. To support the weight of the hanger and to
keep the sub stable during carrier operations, engineers innovated a double hull design. Essentially two hulls stuck together. It gave the mammoth I-400 nearly three times
the displacement of even the largest American submarine. And the I-400 was still a formidable submarine
in the conventional sense. Armed with eight forward mounted torpedo tubes
and a massive deck gun. And to fend off enemy aircraft, three triple-mounted
anti-aircraft guns and a fourth single mounted gun on the sail. But of course the i400’s primary weapons
were its three torpedo dive bombers. The element of surprise was an underwater
aircraft carrier’s greatest advantage. And as the I-400 silently approached its target,
its crew would already begin preparing the aircraft. Mechanics would start by running heated oil
through the aircraft's engines so they would be warmed up and ready to launch. The mammoth submarine would surface a few
hundred kilometers from its target and the race would be on to get three bombers airborne. Each aircraft would be rolled out from the
hangar onto the deck. Crews would then start the engine, unfold
the wings and tail, lock floats into place, and load armament. One by one, the three aircraft would be launched
using a compressed air catapult. The whole process would take thirty minutes. After which the I-400 would dive back to safety
and silently wait for the bombers to return from their mission. The torpedo dive bombers were cutting edge. They were designed specifically for the I-400
and could carry the largest bomb or torpedo in Japan's naval arsenal. Equipped with floats, the aircraft would land
alongside the submarine to be hoisted back aboard using a collapsible hydraulic crane. The aircraft could also be launched without
floats for greater range and performance, but forcing pilots to ditch into the ocean
after their mission. The I-400 was a brilliant design, merging
the stealth of a submarine with the offensive strike capability of an aircraft carrier. But Japan’s new super weapon would make
no difference in the War. On August 15, 1945, after nearly four years
of brutal conflict and with American forces closing in and the bombing of Japanese cities,
Japan finally surrendered. The Americans first intercepted an I-400 off
the coast of Japan two weeks after the surrender. And at first they weren’t exactly sure what
it actually was. The Japanese crew had thrown all of the attack
aircraft overboard. And at first the Americans believed the I-400
was designed to carry cargo. But they’d soon unravel the submarine's
true purpose and why Japan never used them in the War. To start, many in Japan’s Navy considered
Yamamoto’s underwater aircraft carriers to be a farce. And the slow process of launching aircraft
in the middle of a combat zone too dangerous for submarine crews. But resistance to the concept would soon be
the least of Yamamoto’s concerns. Because it took nearly a year to design such
an unconventional weapon. Construction of the first I-400s only began
at the start of 1943. By then, the Japanese were already losing
the War. After a crushing defeat in June of 1942, the
Americans were pushing the Japanese back across the pacific. And Japan was running critically short on
fuel and raw materials. Delaying I-400 construction even further. And Yamamato himself would never live to see
any of his submarines completed. In 1943, while on an inspection tour through
the South Pacific, Yamamoto’s plane was downed by American forces. What started as a plan to build a fleet of
eighteen underwater aircraft carriers was eventually whittled down to just five. And only three were ever completed. The first entering service in 1945. So late in the War that Japan’s military
had already all but collapsed. Launching sneak attacks on American cities
with a handful of dive bombers would’ve been pointless. Even a more strategic mission to bomb the
Panama Canal was abandoned after Japanese command felt that it too would’ve made little
difference so late in the War. The only mission the I-400’s would ever
set out on was a last ditch effort to bomb American forces as they amassed off a tiny
pacific atoll. But as the first I-400s traveled to their
targets, Japan surrendered, finally ending the Second World War. Aircraft carrier submarines had always been
a gamble. A way to change odds so stacked against Japan,
that only through sheer ingenuity could the tables be turned. And had the sub arrived at the start of the
war it might’ve made a difference. But Japan’s secret weapon wasn't without
compromise. The process of launching three aircraft was
supposed to take 30 minutes. But rarely could it be accomplished in less
than 45. A dangerous amount of time for such a large
submarine to be surfaced. And the I-400’s bombers, while sophisticated
in their design were rushed into service and built from lower grade materials due to shortages. They were notoriously unreliable. Rarely could all three get airborne without
some mechanical problem. And the enormous I-400’s depth time, critical
for getting out of danger, was nearly double that of American submarines. Even submerged, it was still vulnerable. With a hull that was riveted, not welded,
it likely would have stood up poorly against depth-charges. Still, the Americans considered the I-400
to be a dangerous weapon. Especially in the wrong hands. And In 1946, with the Soviets demanding to
inspect the subs for themselves, the American’s scuttled the I-400s off the coast of Hawaii
and Japan. Keeping their exact wreckage locations secret,
and closing the chapter on an ambitious new kind of weapon, that in a different set of
circumstances, might’ve changed the course of the war. This video originally appeared on Nebula. A streaming site that I helped create. Nebula is where you can watch Mustard videos
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Incredible feat of engineering, especially given the time constraints but it just seems like there’s too much risk for the reward of bringing only three small bombers to US soil.
Mustard knocks it out of the park again
This vid reminded me of what the History Channel used to be...
The reason this didn't happen is simple Return on Investment.
3 planes per sub versus ~55 planes per carrier
At Pearl Harbor the Japanese strike force consisted of 6 carriers with ~350 planes. To make an equivalent force with subs there would need to be 116 sub carriers.
Lets be generous and say with the element of surprise you'd need half that force. We're still talking about 58 ships versus 6.
The what ifs are fun but the Japanese made the right call, and they still weren't able to match the industry to rival the United States by 1944.
Wow. I wonder what happens to all he designs after the war. Do they just shelve it? No one in the military thinks hey lets see if we can improve some of our weapons.
Ace Combat 7?
This is the kinda stuff I was has back on the history channel.
eh by 1945 nearly all of the Imperial Navy's aircraft carriers were underwater.
Wonder if you make a modern version with a short-takeoff, vertical landing fighters like the F-35B. Probably useless but cool.