The torpedo the Japanese called kaiten was a human-driven suicide bomb, the kamikaze of the seas. Its name translates literally as "return to the sky," which is as descriptive of the driver as it is for its targets.
By 1943, the Imperial Japanese Navy recognized the war was not going its way. This was six months after the decisive Battle of Midway, widely considered to be the turning point in the naval war of the Pacific Theater. The Japanese considered many types of suicide craft, the most well-known and successful are the kamikaze planes, used to great effect toward the end of the war.
The Japanese developed many other kinds of suicide weapons, however. They deployed shinyo suicide boats, fukuryu suicide divers, and human mines. Kaiten suicide submersible torpedoes were more successful than any of these, second only to the kamikaze planes.
Many different types of kaiten were developed, though only the type-1 was ever used in combat. Early designs allowed the pilot to attempt to escape from the torpedo, but since no pilot ever tried, this feature was left off later designs. The sub/torpedo also featured a self-destruct mechanism for the pilot to use if the fuse failed. 300 type-1 kaiten were built, and 100 of those were used in combat.
The Kaiten Type-1 on display in a Japanese museum (wikimedia commons)
The torpedo was a rudimentary submarine, based on the design of a Japanese Type 93 torpedo. It was launched from a submerged submarine and had basic pilot controls and air bottles, all positioned behind more than 1,000 pounds of explosives.
The kaiten kill count numbered more than 187 American troops, the fleet oiler USS Mississinewa in November 1944, a small infantry landing craft (LCI-600), and the destroyer escort USS Underhill in July 1945.