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  • 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.

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