In the last weeks of 1945, a 23-year-old American naval lieutenant stood in the control room of a captured Imperial Japanese Navy submarine in Sasebo, Japan, surrounded by handwritten paper tags. Every valve and switch bore a label, some in English, some in Japanese, because the American crew had no blueprints, manuals, or operating documents. The original crew had destroyed them all before surrendering the vessel. The ship was the I-400, the largest submarine ever built at the time.
She stretched 400 feet, displaced more than three American fleet submarines combined, and carried three folding-wing torpedo bombers in a 115-foot watertight hangar along her spine, launched via compressed air catapult. Her fuel range was an extraordinary 37,500 nautical miles at 14 knots, enough to circle the world one and a half times without refueling. No one in Washington had known she existed until her crew surrendered. After the war, 49 Japanese submarines remained afloat.
Under the terms agreed at Potsdam, all were to be disarmed and disposed of at the Supreme Commander’s direction. Of those, five were selected to be shipped home for study: I-14, I-201, I-203, I-400, and I-41. Preparing the I-400 for the voyage was filthy work. The Americans fumigated every compartment with gas, sealing the boat overnight.
The next morning they swept up dead rats and cockroaches by the bushel. The interior also held surprises: a 25-foot vertical trunk ran from the conning tower straight into the control room so lookouts could drop the entire distance when clearing the bridge, landing on a thick canvas mat. The Americans found the mat so foul they threw it overboard, and an uninformed Japanese sailor who descended that night crashed onto the steel deck, picked himself up, and walked away in silence rather than show anything before the Americans. The boat’s three pressure hulls ran side by side.
Walking the length of the port hull took a man past torpedo rooms, the chief’s quarters, the radio shack, a polished wardroom with a shrine, the control room, engine rooms, and a crew space with varnished raised wooden decking. That entire walk represented only about a third of the vessel. With no machinery histories, watch bills, or spare parts lists, the prize crew improvised, pulling tools and equipment from the tender workshops and abandoned naval stores in the caves around Sasebo. By the end of November 1945, the boat was fit for sea.
Orders called for a trans-Pacific voyage to Pearl Harbor, and nothing more. Commander Joseph McDowell took command. Payne served as executive officer and navigator. The crew aboard was just 44 Americans, four officers and 40 enlisted men, on a boat built for more than 200.
Accounts differ slightly on the voyage’s start, but the record reconciles them: the I-400 left Yokosuka on October 29 under Commander Edward Spruance, and departed Sasebo for Hawaii on December 11. On that second leg, 44 men who had never seen a boat like her four months earlier sailed the largest submarine ever built roughly 5,000 miles across the Pacific without a serious casualty and without taking a pilot at any point. They never once submerged her. The submarine rescue vessel Greenlet accompanied them with deep-sea towing gear in case of trouble, but nothing went wrong.
The crew found the giant boat pleasant, with a dry bridge and an easy roll, and they treated the 130-foot gun platform atop the hangar as a promenade deck. At Guam, the squadron received whistles, a band, and a crowd of curious sailors. The hangar, designed for three attack bombers, now held rifles, bayonets, uniform buttons, rating badges, rubber stamps, and, by Payne’s account, a sampan, all lifted from Japanese stores at Sasebo. The crew opened an informal trade, swapping souvenirs for film projectors, coffee makers, canned hams, and steaks.
A marine officer demanded the return of his motor scooter, and the chief of the boat assured Payne that nobody would ever find such a vehicle on that submarine, which was not quite the same as saying one was not aboard. Later, I-401’s crew arrived at Pearl Harbor with a freshly painted jeep, reportedly purchased by their welfare fund. On January 6, 1946, the three boats came up the channel into Pearl Harbor, dipped their ensigns as they passed the wreck of the USS Arizona, and moored to a Navy band playing on the pier. Payne wanted the boat dived for study.
He had gone through her compartment by compartment with his petty officers and written a list of 39 items needed to make her safe to dive, arguing that the Navy should learn how a hull that size handled underwater and what her constructors knew about big submarines. The answer he received was that decisions had slowed to peacetime tempo and he should stand by. At a submarine officers’ conference in Washington on March 26, 1946, the attendees, including Vice Admiral Charles Lockwood, were told that orders were going out to dispose of all Japanese submarines by sinking. Those in Japan were to be sunk immediately, and those at Pearl Harbor were to be sunk at the discretion of the commander-in-chief, Pacific.
The order gave no reason. The reason that has been consistently given, from NOAA to every standard history of the class, is the Soviet Union. Postwar arrangements gave the Soviets grounds to request access to captured Japanese technology, and they asked for it. Researchers Hackett and Waller, reconstructing the disposal program, put it carefully, writing that the five at Pearl Harbor were reportedly sunk to keep their technology out of Russian hands.
The disposition moved quickly. On April 1, 1946, 24 Japanese submarines left Sasebo under skeleton crews and were sunk off Nagasaki, either by demolition charges or gunfire, in an operation named Roads End. Four days later, several unseaworthy boats were towed out and blown up under a second name, Dead Duck. By October 1946, all 151 boats were accounted for, the great majority on the bottom.
Which brings the story back to the discovery off Barber’s Point, Hawaii, in August 2013. A crew piloting the submersible Pisces 5, working along the seafloor at 2,300 feet with marine archaeologists from NOAA, found a crushed submarine wreck in far deeper water than expected. The forward section had torn away and twisted, torpedo tubes mangled, the hangar and conning tower gone, leaving exposed deck frames and piping. The catapult rail and crane for lifting float planes were still visible, along with the after deck gun.
Identification came by matching the gun, crane, tube arrangement, and running lights against old photographs. What made the find strange was the position. She lay nowhere near the spot the U. S.
Navy had given for the sinking. Researchers had ruled the location out because everything they had pointed further out to sea. Delgado’s reading is that the mismatch was probably on purpose: for a technology denial operation aimed at an ally who was fast becoming something else, a vague position was worth a great deal. No memorandum says so, but there is a gap between the stated positions and the actual wrecks, and circumstances that make the gap easy to explain.
All five of the five Pearl Harbor sinkings were recorded in the paperwork as trials of the Mark 10 exploder, the component that decides whether a torpedo explodes on target. American exploders had a long and bitter history among the men who carried them. The largest boat ever launched spent her last morning afloat was used as a test rig for one. The audience was senior, including Rear Admiral Alan McCann and Commander Lawson Ramage, who wore the Medal of Honor.
The Navy’s own ship history for the submarine that sank the I-400, USS Trumpetfish, describes the event in a single dry sentence, noting her intentional torpedoing of the large Japanese submarine I-400 that year. Nothing anywhere marks the biggest undersea vessel ever built. What she has left is a line in a torpedo trial and a wreck field off Barber’s Point. Payne heard about the sinking later.
Writing to a British submarine museum 39 years afterward, he summed up the end of the boat he had navigated across the Pacific with the observation that at least the torpedoes had worked that time. Anyone who had carried the Mark 14 to sea in 1943 would have caught the joke. In 1947, the U. S.
Navy took a standard fleet boat, USS Cusk, and built onto her after casing a hangar and launching rail. In February of that year, she fired a Loon, the American copy of the German V-1 flying bomb. The program that grew out of that failure produced the Regulus missile, stowed in an airtight deck hangar, arranged so the crew could work on the weapon from inside the pressure hull. By the mid-1950s, American submarines patrolled the Pacific with a flying weapon in a watertight deck hangar and a rail to launch it from, an arrangement very similar to the one the Japanese boat had carried.
Nobody in the American missile program is known to have set foot aboard the Japanese boat. The same arrangement was worked out twice from scratch by people addressing the same problem. Anything that flies must be kept dry and must come out fast. The I-400’s hangar and conning tower are still missing, believed to lie off to starboard of the main wreck in deeper water, torn away in the blunt trauma of three torpedo hits.
They have not been photographed. The hangar, the entire reason the enormous boat was built, is still out there in the dark, a few hundred yards from the rest of her.