I-6 was unique in the Imperial Japanese Navy. Laid down at Kawasaki's shipyard in Kobe on October 14, 1932, and launched in March 1934, she was the sole Junsen II-type submarine ever built. What set her apart was an evolutionary step in naval aviation: while her predecessor I-5 had introduced a hangar for carrying a floatplane, I-6 added a catapult, allowing her to launch aircraft at sea. She was a cruiser submarine, large and long-ranged, designed to operate far from home waters. Commissioned in 1935, she served first in the Second Sino-Japanese War before the larger conflict drew her into the Pacific.

When Japan struck Pearl Harbor in December 1941, I-6 was among the submarines deployed in support of the attack. She went on to torpedo an Allied aircraft carrier and conducted anti-shipping patrols across the Indian Ocean and South Pacific. In March 1943, she laid mines off the Australian coast between Fraser Island and Stradbroke Island -- though her commanding officer lacked good intelligence on Allied shipping routes, and the mines went unnoticed for weeks. They were finally discovered when the Royal Australian Navy sloop Swan, practicing antiaircraft gunnery nearby, triggered at least two detonations that sent columns of water 400 feet into the air. The mines had drifted from where I-6 reported placing them, sliding along the sloping seafloor to a position eight nautical miles offshore.

After the catastrophic Battle of the Bismarck Sea in March 1943, where Allied aircraft destroyed an entire Japanese convoy of eight transport ships and four destroyers, Japan turned to submarines as supply vessels. I-6 was pressed into this unglamorous but essential duty, running supplies between Rabaul and Lae on the coast of New Guinea. Over nine runs between April and May 1943, she delivered passengers, weapons, ammunition, clothing, and food -- stuffing 77 supply drums into her hull on a typical trip. The work was dangerous. Allied motor torpedo boats prowled the waters near Lae, and I-6 encountered them repeatedly, crash-diving to escape on several occasions. On her seventh run, American PT boats fired torpedoes at her while she was on the surface five nautical miles off Lae. I-6 stopped dead, letting the torpedoes pass ahead, then accelerated as another torpedo narrowly missed astern. She submerged, fired a retaliatory torpedo that passed under PT-150's bow without detonating, and slipped away into the dark water.

I-6's war took her to extremes of geography. After her New Guinea supply runs, she redeployed to the frigid North Pacific to support the evacuation of Kiska in the Aleutian Islands, patrolling the Bering Sea with her sister boat I-5. The garrison on Attu had already been annihilated, and the Japanese were determined to save the men on Kiska before the same fate befell them. Between July and September 1943, I-6 completed two patrols in these cold northern waters. She then returned south, resuming supply runs to New Guinea -- this time to Sio on the Huon Peninsula and later to Iboki. On one run she damaged her propellers on an uncharted reef. On another, Allied aircraft attacked while she was still unloading cargo, forcing her to flee with supplies still aboard.

After an overhaul at Yokosuka, I-6 set out on June 16, 1944, bound for the waters off Saipan, where the American invasion of the Mariana Islands had just begun. She never arrived. That very evening, the armed cargo ship Toyokawa Maru -- part of a Japanese convoy returning from the Bonin Islands -- spotted I-6 surfacing and mistook her for an enemy submarine. The ramming was followed by depth charges and machine gun fire. I-6 capsized and sank within minutes, taking all 104 crew members with her. Two weeks later, unaware she was already gone, Japanese headquarters ordered I-6 to rescue Vice Admiral Takeo Takagi from Saipan. When no acknowledgment came, they declared her missing. She was stricken from the Navy list on September 10, 1944. Somewhere in the deep Pacific, roughly 165 degrees west and 19 degrees north, the only J2-type submarine ever built rests on the ocean floor -- sunk not by the enemy she had fought across half the world, but by her own side.