Sunken WWII Imperial Japanese Navy I-400 Class “Super Sub” Located Off Hawaiian Coast of Oahu

This realistic painting of the I-400 shows that is was truly a MASSIVE vessel. While you're looking at this image remember—it's longer than a football field! (Image: Harry Cooper)

Japanese Imperial Navy I-400 submarine with aircraft hangar and launching ramp. (Image: Harry Cooper)

In this WWII image, an American sailor inspects the sub's massive, open hanger door that leads to the interior of the ship where the bomber seaplanes were stored. (Photo: US Navy)

In this WWII photo, an American sailor inspects a captured I-400 sub’s massive, (open) watertight door that leads to a distinctive round hangar where 3 bomber seaplanes (with special folding wings) were stored. (Photo: US Navy)

Longer than a football field, “underwater aircraft carriers” scuttled by US Navy to keep secret from Soviet Union

Imagine if you will, during WWII, having the ability to surface a massive submarine anywhere off the coastline of an enemy nation, quickly launch 2 or 3 bomber aircraft which could fly fast and low so as to avoid easy detection, attacking any designated target and then retrieving the planes and slipping silently beneath the waves to make good your escape.

If successful and frequent, such “stealthy” attacks would have struck terror in the hearts of any nation and possibly altered the outcome of the war. Not surprisingly, the navies of many countries, both Axis and Allied, were experimenting with such “underwater aircraft carriers” and similar deadly innovations. But of them all, by the end of the war, the Japanese Imperial Navy had achieved the greatest success rate, having built 3 of the fearsome, deadly, and game-changing “Super Subs.”

The painting on this model box shows how the I-400 would have looked at speed on the surface, after having launched its 3 bomber seaplanes. (Art: Tamiya Models)

The painting on this model kit box shows how the Japanese I-400 sub would have looked while running at speed on the surface, and after having launched its 3 bomber seaplanes. (Art: Tamiya Models)

In this undated, captured WWII photograph, Japanese submariners can be seen crouching for cover as one of their bomber seaplanes is launched by catapult, somewhere in the Pacific. (Photo: US Navy)

In this undated, captured WWII photo, Japanese submariners can be seen crouching from the prop-wash of a bomber seaplane as it is launched by catapult off the ship’s deck, somewhere in the Pacific. (Photo: US Navy)

The Enemy Below—Discovered!

Editor: I know I’ve “buried the headline” somewhat today, but here’s where we were headed:

In a surprise announcement made last week by the Hawaii Undersea Research Laboratory at the University of Hawaii, it was revealed that one of the three known I-400 submarines, which had been scuttled by the US Navy after the war, has been discovered—again—at the bottom of the ocean—just off the coast of the island of Oahu. It is yet another startling WWII-related find, the latest in a string of such finds (many of them reported here on The Joe Report) concerning lost, crashed or sunken warships and military aircraft. According to CNN:

“The I-400 submarine was discovered in 2,300 feet of water off the southwest coast of Oahu. ‘Finding it where we did was totally unexpected. All our research pointed to it being further out to sea,’ said Lab Ops Director and Chief Sub Pilot Terry Kerby.”

This authentic (color-enhanced) B&W WWII photo of an I-400 sub at sea gives an idea of just how long and imposing they were. Absolutely breathtaking! (Photo: Rtas Vaduum)

This authentic (color-enhanced) B&W WWII photo of an I-400 sub at sea gives an idea of just how long and imposing these Japanese warships were. Absolutely breathtaking! (Photo: Rtas Vaduum)

Aptly named “Super Subs” Remained Unparalleled in Size Until 1960s

It’s also important to remember that at the end of WWII, there was nothing as large as the I-400 submarines (operating underwater) anywhere on the globe. These massive vessels made the vaunted “Wolf Pack” U-Boats of the Kreigsmarine (German Navy) look absolutely puny by comparison (see previous story on the famous U-505 HERE). The CNN article went on to say:

“At nearly 400 feet long, the I-400 and its two sister ships were the largest submarines ever built before the nuclear age. Initially conceived as a weapon to target the U.S. mainland and capable of reaching any point on the globe without refueling, the subs were effectively underwater aircraft carriers outfitted with three folding-wing seaplanes capable of carrying an 1,800-pound bomb.”

This WWII photo shows the battle-scarred deck and conning tower of a captured Imperial Japanese I-400 class submarine, with its "game-changing" water-tight hangar door (located just forward and below the deck gun) shown in its surfaced, open position. Bomber seaplanes (stored inside) would exit, attach to a catapult, and launch from the sub to attack targets both at sea and on land. For retrieval, the aircraft would rendevoux with the sub and be lifted aboard with the ship's crane (not shown). (Photo: US Navy)

This rare closeup WWII photo shows the hangar and conning tower of a captured Imperial Japanese I-400 class submarine, with its “game-changing” water-tight hangar door (located just forward and below the deck gun) in its surfaced, open position. Bomber seaplanes (stored inside) would exit, attach to a catapult, and launch from the sub to attack targets both at sea and on land. For retrieval, the aircraft would rendezvous with the sub and be lifted aboard with the ship’s crane (not shown). (Photo: US Navy)

This revealing bow and stern view of the I-400 class submarine shows how its conning tower was constructed in a very similar way to that of a traditional aircraft carrier. (Image: subcommitte.com)

These bow and stern views of the I-400 reveals how its conning tower was constructed in a way similar to that of a traditional aircraft carrier. (Image: subcommitte.com) Click to enlarge.

Imagine the damage the I-400 submarines COULD have done to Allied interests had they had more time or been produced in greater numbers. But as the CNN article revealed:

“The ships were never used to attack the mainland United States and saw only limited service before Japan surrendered in 1945. But their novel design represented a tactical shift in thinking about the use of submarines, which until then had been strictly seen as anti-ship weapons.”

This WWII photo showing the sub's crew massed up on deck, provides an even better idea just how BIG these Japanese "Super Subs" really were. (Photo: Rtas Vaduum)

This WWII photo showing the entire ship’s complement amassed up on deck, provides an even better idea just how MASSIVE these “underwater aircraft carriers” really were. (Photo: Rtas Vaduum)

What we find especially interesting is that after almost 70 years, the (re)discovery of this submarine is still considered somewhat of a sensitive subject, requiring the University of Hawaii researchers to proceed carefully and slowly before announcing their findings to the world. According to CNN:

“The sub was found in August, but the lab didn’t notify the public (in November) until after informing the US State Department and the Japanese government. With tensions rising between the Soviet Union and the United States after the war, the US Navy scuttled the ships to avoid their advanced technology falling into the hands of the Soviet Navy in what would become one of the first intrigues of the Cold War.”

In one of the last photos of an I-400 taken before its heading out to sea (and subsequent capture by the US Navy), this peaceful sunset image (colorized B&W) provides a good view of the sub's unusual round aircraft hangar and a few sailors for a sense of scale. (Photo: Rtas Vaduum)

This peaceful sunset image (colorized B&W) provides a good view of the sub’s unusual round aircraft hangar and a few sailors up on deck provide a sense of scale. (Photo: Rtas Vaduum)

I-400: Japan's Secret Air Strike Submarine, by Henry Sakaida, et. al, (Photo: Hikoki Publishing)

I-400: Japan’s Secret Air Strike Submarine
(Photo: Hikoki Publications)

Bottom Line: What an amazing discovery! And there’s so much MORE to this story than we could possibly hope to cover here. For example, did you know that one of the 3 Super Subs had been tasked with destroying the Panama Canal, thereby greatly slowing Allied ship movements to and from the Pacific? And…that one of the subs was even painted in US markings for its final mission? What happened to them all? If you’d like to find out, we highly recommend reading “I-400: Japan’s Secret Air Strike Submarine,” a 2006 book by Henry Sakaida, et. al, that can be purchased in bookstores and on Amazon HERE.

Finally… if you’re a submarine fan or WWII history buff (and who isn’t?), we highly recommend viewing the excellent (1-hour) “Super Subs” video that we found over on YouTube (click on link HERE). Enjoy—and “Dive! Dive! Dive!”

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4 thoughts on “Sunken WWII Imperial Japanese Navy I-400 Class “Super Sub” Located Off Hawaiian Coast of Oahu

  1. Joseph Benedetto says:

    Super awesome~! The 400 class subs were absolutely amazing. I can only ponder what it would be like to recover and refloat it….

  2. Joseph Benedetto says:

    The Japanese proved the aircraft-carrying submarine concept when the I-25 sailing off our NW Pacific coast torpedoed a Canadian freighter, threaded the minefields at the mouth of the Columbia River and then shelled one of our coastal forts — and on their next patrol, flew a bombing mission on the American mainland. (And it did this with only one bomber. Imagine what an I-400 could have done with three.)

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