After 70 Years, the Search for Japan's Lost Mega-Submarine Is Over

A Japanese super submarine has been found lurking in the deep—right where American forces scuttled it nearly 70 years ago.

On August 1, a team of underwater researchers piloting a submersible investigated a large object picked up on radar off the coast of Oahu in the Hawaiian Islands. As pilot Terry Kerby of Hawaiʻi Undersea Research Laboratory (HURL) brought the Pisces V submersible closer to the massive object, what could easily have been mundane instead revealed itself as I-400, a legendary World War II sub that opened a new era of submarine strategy.

The find of the long-lost submarine—which, at 400 feet long, was far larger than any American contemporary—has only been revealed this week after the National Oceanic and Atmospheric Administration, which funded the research, consulted with the US state department and Japanese officials. It's the first time since 1946 that humans have seen the incredible submarine, one of only a handful of sen-toku ("special submarine") constructed for the Japanese Imperial Navy at the end of World War II.

“The I-400 has been on our ‘to-find’ list for some time. It was the first of its kind of only three built, so it is a unique and very historic submarine,” said Kerby in a release.  “Finding it where we did was totally unexpected. All our research pointed to it being further out to sea. The multi-beam anomalies that appear on a bottom survey chart can be anything from wrecks to rocks—you don’t know until you go there. Jim and Hans and I knew we were approaching what looked like a large wreck on our sonar. It was a thrill when the view of a giant submarine appeared out of the darkness.”

After the war, Japan surrendered it and its sister ship, I-401, to American forces. The US had a serious interest in investigating the sub, which was technologically stunning for far more than just its size.

According to military reports, the diesel-electric sub had a range of 37,500 miles, enough for a planned trip to attack the Panama Canal, and far more than any other submarine until nuclear-powered subs arrived a decade or so later. The ship could carry a trio of folding-wing bombers, a capability that's marked as the first time submarines had a focus beyond simply attacking ships, and which is a precursor to the missile subs that defined Cold War submarine strategy.

“The I-400 is technologically significant due to the design features associated with its large watertight hangar,” said Dr. James Delgado, director of NOAA’s Maritime Heritage Program, who was with Kerby on the I-400 dive. “Following World War II, submarine experimentation and design changes would continue in this direction, eventually leading to ballistic missile launching capabilities for US submarines at the advent of the nuclear era.”

So how does something so large get lost? After investigating the ship, and fearful the Soviet Union might try to learn its secrets, the US scuttled the ship in 1946. At the time, the US military only said that five subs had been sunk during target practice, and that it couldn't be sure where it was. Since then, finding the long-lost ship has become a priority for historians and underwater explorers, as it's a huge piece of history that's simply rotting away.

But finding the ship remained elusive for years. According to the New York Times, the official position listed for the sub wrecks was miles from the actual position, but during years of exploration dives with HURL, Kerby kept a record of interesting objects on the ocean floor that might be worth later exploration. While most of them ended up being rocks or other geological features, the August 1 dive picked up something more.

“As we approached it from the sonar the excitement built, and suddenly there you were, at the mangled bow of a submarine,” Dr. Delgado told the Times. “But it wasn’t readily apparent that we were on I-400 until we really started to go through it piece by piece and match things up."

It's hard to envision what it must be like to see a giant mystery sub appear from the deep, but at least we get to join along with the HURL video. For military historians and archaeologists, it's the answer to a very old question about a truly influential submarine—one that, due to resource constraints, never had the influence or numbers the Japanese intended. Although research may continue, there are no plans to try to salvage the sub. Now that I-400 has been found, it will be left to rest.

@derektmead