Until the 1960s, Japan's three I-400-class subs were the largest submarines ever built. They were so large, in fact, that they could each carry and launch three Aichi M6A Seiran amphibious aircraft. The idea was that the submarines could stealthily bring the planes to within striking distance of US coastal cities, where they could then take off and conduct bombing runs. Now, for the first time since it was scuttled at the end of World War II, one of the sunken subs' aircraft hangars has been photographed.

The Seiran bombers, which were designed specifically to be launched from submarines, were kept in the hangars with their wings folded up while the vessel was in transit. When it was time to launch, the sub surfaced and each plane was pushed out and attached to an 85-foot (26-meter)-long compressed air-powered catapult on the forward deck. They could be equipped with pontoons for retrieval after their missions, although they could also be ditched at sea and their pilots rescued by the submarine crew – additionally, at least one kamikaze mission was planned.

Fortunately for the Allied countries, the three 400-ft (123-m)-long submarines – I-400, I-401 and I-402 – were built near the end of the war, and never got to make any attacks.

An Aichi M6A Seiran amphibious bomber

After Japan was defeated and the vessels were captured, the Soviets demanded access to the subs. To keep that from happening, American forces transported the submarines to a location off the southwest coast of the Hawaiian island of Oahu, where they were torpedoed and sank in 1946.

In December of 2013, a Pisces V manned submersible from the US National Oceanic and Atmospheric Association (NOAA)'s Hawaii Undersea Research Laboratory found the wreck of the I-400, lying over 2,300 feet (701 m) below the surface. The submarine's cylindrical aircraft hangar was no longer attached, however, and couldn't be located.

One of the I-400-class subs' hangars, being inspected by American forces

More recently, Japanese broadcaster NHK funded another dive, working with NOAA to try and photograph the hangar. As can be seen in the video below, it succeeded. The team also located the submarine's conning tower for the first time, along with its bell.

"The waters off Hawaiʻi not only encompass an important part of Native Hawaiian culture, but are also a veritable museum of our maritime past," says James Delgado, director of NOAA's Maritime Heritage Program. "As America’s ocean science agency, we’re committed to working with partners like HURL [Hawaii Undersea Research Laboratory] and NHK to learn more, and to share more of what lies beneath the waves."

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