Researchers from HURL have been using manned submersibles since the early 1990s to search for submerged subs and other historic wrecks. According to Terry Kerby, HURL operations director and chief submarine pilot, ““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.” Kerby and his team came upon the sub accidentally while searching for other wrecks; their research had indicated the vessel was further out in the ocean.

The Japanese developed the I-400 in the early 1940s for the purpose of using submarine-launched aircraft to make aerial attacks against the East and West coasts of the United States. The I-400 could carry three folding-wing M6A1 Seiran bombers in its watertight, on-deck hangar. Each plane could be launched by catapult from the sub shortly after it resurfaced. Prior to the I-400, submarines were used primarily for attacks against other ships.

The I-400 trained for one mission, a strike against the U.S.-controlled Panama Canal. By attacking the canal, the Japanese believed they could disrupt U.S. supply lines to the Pacific. Ultimately, the attack never took place, and at the end of the war, the I-400, along with four other Japanese subs, was captured by the U.S. Navy and transported to Pearl Harbor for inspection. When the Soviet Union said it wanted to examine the subs, under the terms of a treaty that ended the war in the Pacific, the U.S. torpedoed and sank the vessels off Oahu in 1946, at the onset of the Cold War, in an effort to prevent the technology from ending up with the Soviets. The Americans claimed the subs had been used for target practice and said they had no knowledge of the vessels’ exact locations.

The HURL team located the I-400 in August 2013, but the discovery was kept under wraps until it could be reviewed with U.S. state department officials and the Japanese government. To date, HURL has found four of the five scuttled Japanese subs.

As of now, there are no plans to move the wreckage of the I-400 from its current site.