Just after noon on 8 June 1943, the battleship Mutsu exploded while at anchor in the Hashirajima fleet anchorage in Japan’s Inland Sea, roughly two miles off the coast of the home islands. The ship broke in two and sank within minutes. There was no enemy within thousands of miles, no aircraft overhead, and the water was flat calm. The Mutsu was a Nagato-class battleship of about 40,000 tons at full load, commissioned in 1921.
She had been one of the Imperial Japanese Navy’s proudest symbols, the first capital ship in the world to mount 16-inch guns, and she had served as the emperor’s showpiece at naval reviews through the 1920s and 1930s. The year before her loss, she had been present at the Battle of Midway under Admiral Isoroku Yamamoto, though she never fired her main battery in that battle. Yamamoto himself had used her as his afloat headquarters in April 1941 while planning for the Pacific War, but he was killed over Bougainville in April 1943, just weeks before his former flagship went down. The blast came from beneath the third turret, detonating the powder magazine below it.
The forward 535 feet of the ship sank almost immediately. The aft section, about 147 feet, remained afloat and burned until the early hours of 9 June. Of the 1,474 men aboard, 1,021 died, including the captain, Miyoshi Teruhiko, and 53 visitors from the Sasebo Naval Air Group. Those visitors were flight cadets and instructors who had come aboard for a day of familiarization with a capital ship, part of a long-standing Navy practice of sending air personnel to ride the big warships.
On the day of the explosion, 113 cadets and 40 instructors were aboard; only 13 survived. Nearby ships, including the battleship Nagato, the carrier Junyo, the heavy cruiser Mogami, a light cruiser, and two destroyers, raced to the scene and pulled survivors from the burning water. Then the Imperial Japanese Navy began a cover-up that would last for decades. The sinking was declared a state secret, and mass cremations began that very afternoon on the nearby island of Suo Oshima, before bodies could be identified.
Captain Miyoshi’s body was recovered on 17 June, but his wife was not told he was dead until 6 January 1944, nearly seven months later. The 353 survivors were split up and posted to garrisons across the Pacific, including Truk, Saipan, Tarawa, Makin, and Kwajalein. The choice of posts was not random: these were the islands the Navy expected the Americans to attack next. Most of Mutsu’s survivors died in those island assaults during 1944, taking what they had witnessed with them.
The deception even reached the Navy’s own communications. Mutsu’s call sign stayed on the air, with other ships transmitting under it to fake her presence in fleet traffic. When divers were sent to survey the wreck, they were not told what they were looking at. They were told only that the wreck resembled the sister ship Nagato and that they should familiarize themselves with Nagato’s layout first.
The Navy did not trust its own divers to know that Mutsu was on the bottom. The deception fooled the enemy as well. In March 1944, nine months after the sinking, American naval intelligence intercepted a Japanese prisoner of war report claiming Mutsu had been torpedoed in home waters. The intercept, decoded by the Australian-American FRUMEL unit, shows that the United States Navy believed into 1944 that Mutsu had been damaged by a submarine and had limped home.
An official investigation opened within days of the disaster. Admiral Shiozawa Koichi convened what was called the M Commission, assisted by Rear Admiral Hoshina Zenshiro, who had once commanded the Mutsu herself. The commission issued its preliminary report on 25 June 1943, before divers had even finished surveying the wreck. The report blamed a single man: a gunner’s mate assigned to the third turret who had been due in a naval court the day of the explosion to face charges of petty theft.
The theory was that he had set a fire in the powder magazine as a diversion to escape the trial, and that the fire had reached the propellant. There was one immediate problem with the theory: the suspect’s body was not found in the wreck. The commission also tested alternatives. The third turret’s powder magazine held Type 3 “sanshiki dan” shells, anti-aircraft rounds designed to burst in the air and scatter incendiary tubes.
Samples taken to the Kamegakubi proving ground would not explode, and chemists reported the propellant would not ignite on its own below 80 degrees Celsius. Survivors described a column of brown smoke; Type 3 shells produce white smoke, but propellant produces brown. The chemistry pointed one way, but the commission pointed the other. The verdict held for the duration of the war.
The Navy never announced the sinking. The public was never told. Families of the dead were notified that their sons had died in action in waters the Navy would not name, on a date the Navy would not specify. Later theories emerged in the decades after the war.
British researcher Mike Williams, writing in the journal Warship in 2009, argued that a fire in an adjacent compartment had ignited black powder primers stored near the shells, which then set off the main propellant charge. A Japanese naval constructor suspected a British intelligence agent, while an engineer argued for static electricity in the Type 3 shells. None of these theories was ever confirmed. The wreck lay in about 35 meters of water for 27 years.
In 1970, the Fukada Salvage Company began an eight-year operation to raise it. On 23 July 1970, the crew lifted the fourth turret, the one immediately aft of the blast. Inside, in the compartment beneath the gun house, lay the remains of a man. He was identified by two personal seals, the small carved stamps carried in a pocket.
The seals belonged to the gunner’s mate the commission had blamed in 1943. He had not escaped. He had died in the very turret he was supposed to have sabotaged, at the station he had manned for months, with no sign of a diversionary fire. The salvage continued.
The aft section was raised in March 1971, and the forward section, 535 feet of battleship, was lifted in February 1972 with a 1,000-ton floating crane. By the time the operation ended in 1978, the crew had recovered 809 bodies. Other artifacts had gone elsewhere. The Imperial Chrysanthemum Crest from the ship’s front, a brass fixture nearly four feet across, had gone to a private collection after a different operator raised it in 1953.
A secondary battery casemate gun reached Yasukuni Shrine in Tokyo in 1963. In November 1972, the Mutsu Memorial Museum opened in the town of Tawa on Suo Oshima, the island nearest the wreck. The museum holds hull plating, a bronze propeller, and personal effects recovered from the mud inside the hull. It is a small museum, not on the standard tourist route, and is staffed in part by descendants of the men who were aboard.
The M Commission’s report, which blamed a dead man who had actually died at his station, was never revisited. The Imperial Japanese Navy ceased to exist in 1945, dissolved under American occupation, and the government that succeeded it did not reopen the file. The official cause of the explosion remains in the formal record what the commission said it was in 1943. The comparison with another battleship loss was stark.
When the Hiei went down in 1942 after a surface engagement with American cruisers and destroyers off Guadalcanal, the Navy announced the loss and reported the dead. Mutsu, seven months later, destroyed herself in her own fleet anchorage with no enemy within a thousand miles. Some 1,121 men died. The Navy decided the difference between the two sinkings was the difference between a story that could be told and a story that could not.
What remains of the wreck sits at the bottom of Hashirajima, a section of hull in about 30 meters of water, with its highest point about 12 meters below the surface. The water is clear, the site is marked, and divers still visit it today. The survivors who came home in time returned to a country that had never been told what they had survived, and most of them never said.