One of the craft was the largest non-nuclear sub ever built and had the ability to circle the globe 1 1/2 times without refueling. Called the I-14, the behemoth was 400 feet long and 40 feet high and carried a crew of 144. It was designed to launch two folding-wing bombers on kamikaze missions against U.S. cities such as New York and Washington, D.C., although changes in tactics, and the end of the war, prevented such attacks.
"In their time, they were very revolutionary," said retired Col. Robert D. Hackett, a military historian with CombinedFleet.com, an online collection of information about the Imperial Japanese Navy. He was not involved in the new find. "We were quite interested in the technology."
The two subs are among five that were captured at war's end and brought to Hawaii, then sunk off Oahu after U.S. technicians had gleaned all their secrets. One of the five, the I-401, which carried three aircraft, was discovered on St. Patrick's Day in 2005, but the search for the others had proved futile.
But the news of that find stimulated Navy veterans to contact the search team with more information, said Terry Kerby, operations director at the Hawaii Undersea Research Lab, a University of Hawaii research center funded by the National Oceanic and Atmospheric Administration.
The team was put in contact with Charles Alger, who was chief mate on the crew that sailed the I-14 to Hawaii.
Alger, who died recently, filmed the sinking of the ship for U.S. naval intelligence. When he played the footage for Kerby's team, researchers were able to triangulate from landmarks on the shore and locate the approximate site of the sinking.
"It wasn't anywhere near the other sites" where they had been searching, Kerby said.
In February, Kerby piloted the manned submersibles Pisces IV and Pisces V to the ocean floor in the region.
"The first dive, we came across a giant bow with a catapult launch ramp, then the main hull" of the I-14, he said. As the submersible was moving away, he observed another big sonar contact "right at the end of the dive, so we knew there was another one down there. It was very propitious to find the two together."
The Pisces submersibles are dedicated to scientific expeditions, such as exploring geothermal vents, and are not normally used for wreck-hunting. But before every scientific season begins, the team makes a series of test dives, typically three. The team uses these to look for submerged wrecks. The team has found not only the I-401, but also Japanese mini-subs and other wrecks of less historical significance in the waters off Hawaii.
This weekend, he will do another series of test dives, Kerby said, and will look for the two remaining subs.
"What really jumps out is these three submarines [found so far] represent quite an advanced concept of military technology that was ahead of its time," said archaeologist Hans Van Tilburg of NOAA's Office of National Marine Sanctuaries, who was part of the team.
Earlier subs had carried reconnaissance planes, but the I-400 series -- which included the I-14 -- carried either two or three Achii Seiran bombers, whose wings and tail could be folded to fit into the sub.
Most such planes were destroyed, but a restored Seiran is on display at the Smithsonian Institution's Udvar-Hazy Center at Dulles Airport in Washington. The I-400 series had a range of 37,500 miles, allowing it to operate anywhere in the world without support.
In effect, they were stealth weapons, able to pop up off the coast, launch their planes in 7 to 10 minutes, then disappear underwater again, Van Tilburg said.
The initial Japanese plan was to use the I-400 subs to target New York and Washington. That idea was dropped, and the Japanese military decided instead to fit them with torpedoes to bomb the locks at the Panama Canal.
Ultimately, it was decided to send them against the U.S. fleet anchored at Ulithi Atoll, north of the Philippines, the staging area for the proposed invasion of Japan.
But Japan's surrender came a day before the scheduled submarine attack.
The commander of the squadron, Capt. Tatsunosuke Ariizumi, ordered all the planes and torpedoes to be launched and sunk, then ordered the subs to be scuttled with all hands aboard.
Cooler heads prevailed and the subs were later captured -- although Ariizumi committed hara-kiri because he felt that he was disgraced.
Alger led a team that boarded the I-14 with only a tommy gun and a pistol and negotiated a peaceful surrender.
The I-201 attack sub carried "a hellacious number of batteries" that enabled it to achieve a submerged speed of close to 20 knots, said Hackett, the military historian. U.S. subs could manage only about 8 knots.
The I-201 also had a double-hulled construction that allowed it to safely submerge to 300 feet -- compared with 200 feet for U.S. subs -- and a rubberized coating that muffled internal noises and that partially absorbed radar from enemy ships.
Because of that coating and the low temperatures at the sub's 2,500-foot-deep resting place, Kerby said, the vessel is remarkably well preserved. The team has no plans to retrieve any artifacts from the vessels.
But the Japanese were not ahead of the U.S. in every way, Van Tilburg said. Their radar, for example, was inferior.
The dives were partially supported by the National Geographic Channel, which will show a film about them, "Hunt for the Samurai Subs," Tuesday.