Otsushima Kaiten Base

Otsushima Kaiten Base (大津島回天基地) was the Imperial Japanese Navy's primary facility for testing, training, and launching kaiten (回天) — manned suicide torpedoes designed to reverse Japan's deteriorating war fortunes. Located on Otsushima (大津島), a small island in the Seto Inland Sea accessible by ferry from Tokuyama Port in Shunan, Yamaguchi Prefecture, it was the first of four kaiten bases established across Japan.

The island had served as a naval torpedo launch testing site since the late 1930s (accounts give 1937 or 1939), with facilities including a large shallow bay ideal for observing test firings. When the kaiten program was formally approved as a naval weapon in August 1944, the existing infrastructure was rapidly converted to support the new suicide torpedo operations. The base officially opened on September 1, 1944, with training commencing on September 5.

The kaiten (literally "turn the tide of heaven") was conceived by Lieutenants Hiroshi Kuroki and Sekio Nishina in late 1943, and the Navy ordered a prototype in February 1944 as Japan faced mounting losses across the Pacific. The weapon was essentially a modified Type 93 Long Lance torpedo fitted with a cockpit, periscope, and steering controls — a pilot would board the weapon aboard a submarine, launch toward an enemy vessel, and guide it to impact at the cost of his own life. Trials of the prototype were held on July 25, 1944, at the Dainyū (大入) launch range, and the weapon was formally adopted on August 1.

The base's physical infrastructure centered on a 250-meter tunnel cut through solid rock, through which kaiten were transported on rail carts between the maintenance area and the launch platforms on the seaward side. The launch facility consisted of 8 large caissons manufactured in Oita Prefecture and 5 smaller caissons built locally on Otsushima, forming concrete platforms from which the torpedoes were lowered into the water. Supporting facilities included a torpedo adjustment factory, repair and machining shops, cranes, classrooms, an electricity transformer station, hazardous materials storage, a firing test evaluation office, observation stations overlooking the test area, kitchens, barracks, and worker dormitories.

Most kaiten volunteers were in their early twenties, though some were as young as seventeen. They trained in unarmed versions of the weapon, learning to operate the controls, navigate using the periscope, and execute attack runs around the island's waters. Between November 1944 and August 1945, kaiten were deployed operationally from submarines against Allied naval targets across the Pacific. Of the four kaiten bases nationwide — Otsushima, Hikari, Hirao (both also in Yamaguchi Prefecture), and Oga in Oita Prefecture — 87 kaiten pilots died on combat sorties, 15 in training accidents and 2 by suicide at the war's end (some counts give a total of 106); including maintenance crews and others, the toll reaches 145.

In November 2006, the Japan Society of Civil Engineers designated the former launch training base as a Civil Engineering Heritage Site (選奨土木遺産), recognizing it as a valuable and rare surviving example of wartime military infrastructure. Otsushima remains the only location in Japan where the original kaiten base facilities survive substantially intact, making it an irreplaceable physical record of this desperate chapter in the Pacific War.