Bando remembered Captain Ariizumi becoming impatient with the surrender negotiations, preferring to scuttle the submarine and have the officers and crew commit suicide. Johnson was also concerned about the possibility of mass suicide aboard the sub, but after some haggling, terms were agreed upon and a prize crew from Segundo boarded I-401, checked that there were no torpedoes left, chained the hatches open to prevent the sub from diving and accompanied it on its return to Japan.

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At 0500 hours on August 31, the U.S. flag was hoisted aboard I-401 and Commander Nambu delivered two samurai swords as a symbol of surrender to Lieutenant J.E. Balson, Segundo’s executive officer and prize crew chief. Shortly thereafter, Ariizumi shot himself in his cabin with a pistol; his body was subsequently buried at sea. “It was a small boat,” Asamura said. “Everyone knew the commander had killed himself.”

Nambu recalled that the officers and crew of I-401 “received gentle treatment by the U.S. Navy after the surrender.” Bando noted that Johnson even invited him to visit the United States after the war.

Escorted by Segundo, I-401 sailed to Yokosuka in Tokyo Bay, where it officially surrendered to the U.S. The sub was stricken from the Imperial Japanese Navy’s active duty roster on September 15.

The I-400 submarines only saw eight months of service from their launch to their surrender, and the Seirans likely never flew in combat. But the U.S. Navy was so impressed by the underwater aircraft carriers that it decided the subs merited further study. On December 11, 1945, I-400 and I-401 sailed with an American prize crew of four officers and 40 enlisted men (as well as a load of smuggled Japanese war souvenirs in I-400’s hangar) from Yokosuka to Pearl Harbor. They were escorted by a sub rescue vessel, and after an uneventful trip arrived in Pearl on January 6, 1946.

According to the late Thomas O. Paine, who served as executive officer and navigator during I-400’s trip to Pearl Harbor, the absence of manuals for the I-400s did not stop American crews from figuring out how to operate the subs because “Japanese submarine design…followed fairly standard practice.” In an unpublished memoir, Paine wrote that the prize crews developed their own drawings and color codes for I-400’s operating systems as well as “learned under the critical eyes of Japanese petty officers.”

Paine explained that I-400’s interior included a “large torpedo room, chief’s quarters, radio shack, capacious wardroom featuring fine wooden cabinet work, a Shinto shrine, officer’s staterooms, and a large control room.” He also described the sub’s aft crew compartment as having “raised wooden decks polished like a dance floor—you took your shoes off before walking there.”

Both subs were extensively studied at Pearl, though the Navy never tried submerging either one. When the Soviets asked for access to the I-400s as part of an information-sharing agreement, U.S. officials decided to prevent them from obtaining potentially disruptive technology by scuttling the submarines. I-402 was sunk off Japan’s Goto Island in April 1946, and I-401 was tor­pedoed by the submarine Cabezon and sunk off Pearl Harbor on May 31. I-400 quickly followed it to the bottom.

In March 2005, the Hawaii Undersea Re­search Laboratory, using two deep-diving submersibles, located I-401 off the coast of Kalaeloa in 2,665 feet of water. The main hull sits upright on the bottom. The bow is broken off just forward of the airplane hangar, and the “I-401” designation is still clearly visible on the conning tower. Otherwise the sub appears in remarkably good condition. I-400 and I-402 have yet to be found.

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