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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