Researchers Discover Long-Lost WWII Aircraft-Carrying Submarine Off Oahu

U.S. Navy photo shows the open plane hangar of the I-400 submarine.

Researchers at the University of Hawaii this week have confirmed the discovery of a long-lost World War II-era Japanese mega-submarine, the I-400, lost since 1946 when it was intentionally scuttled by U.S. forces after its capture, ending a decades-old Cold War mystery of just where the lost submarine lay.

The Hawaii Undersea Research Laboratory (HURL) at the University of Hawaii say they discovered the 1-400 in August off the southwest coast of Oahu in waters about 2,300 deep and are releasing details of their findings now following a review by NOAA with the U.S. state department and Japanese government officials.

At 400 feet long, the I-400 was known as a “Sen-Toku” class aircraft-carrying submarine, the largest submarine ever built until the introduction of nuclear-powered subs in the 1960s. With a range of 37,500 miles, the I-400 and its sister ship, the I-401, were able to travel one and a half times around the world without refueling, a capability that has still never been matched by any other diesel-electric submarine.

The subs discovery was led by veteran undersea explorer Terry Kerby, HURL operations director and chief submarine pilot, as part of a series of dives funded by a grant from NOAA’s Office of Exploration and Research and the University of Hawai‘i at Mānoa’s School of Ocean and Earth Science and Technology (SOEST).

“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. “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.”

The I-400 and the I-401 aircraft-carrying submarines held up to three folding-wing float-plane bombers that could be launched by catapult just minutes after surfacing. Each aircraft could carry a powerful 1,800-pound bomb to attack the U.S. mainland, although neither was ever used for its designed purpose, their missions curtailed by the end of armed conflict in the Pacific.

“The I-400 is technologically significant due to the design features associated with its large watertight hangar,” Delgado said. “Following World War II, submarine experimentation and design changes would continue in this direction, eventually leading to ballistic missile launching capabilities for U.S. submarines at the advent of the nuclear era.”

At the end of WWII, the U.S. Navy captured five Japanese subs, including the I-400, and brought them to Pearl Harbor for inspection. When the Soviet Union demanded access to the submarines in 1946 under the terms of the treaty that ended the war, the U.S. Navy sank the subs off the coast of Oʻahu, claiming to have no information on their precise location, in an effort keep their advanced technology out of Soviet hands during the opening chapters of the Cold War. HURL has now successfully located four of these five lost submarines.

The HURL crew identified the wreck site by combing through side-scan sonar and multi-beam sonar data to identify anomalies on a seabed littered with rocky outcrops and other debris. The wreck was positively identified as the I-400 based on features including its aircraft launch ramp, deck crane, torpedo tube configuration, and stern running lights. The remains of the submarine’s aircraft hangar and conning tower appear to have been separated from the wreck, perhaps in the blunt trauma of the three U.S. Navy torpedo blasts that sunk the ship in 1946.

“These historic properties in the Hawaiian Islands recall the critical events and sacrifices of World War II in the Pacific, a period which greatly affected both Japan and the United States and shaped the Pacific region as we now know it,” said Van Tilburg, maritime heritage coordinator for NOAA in the Pacific Islands region. “Our ability to interpret these unique weapons of the past and jointly understand our shared history is a mark of our progress from animosity to reconciliation. That is the most important lesson that the site of the I-400 can provide today.”

Initial Discovery

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