The Fukuryu, or “Crouching Dragon” in Japanese, refers to a special attack unit of Japanese soldiers prepared to resist the Allied invasion of Japan’s home islands.

These Suicide divers or Kamikaze frogmen (‘Kamikaze’ means ‘divine wind’) would be armed with a Type 5 mine, which carries a 33 Ib (15kg) explosive fitted to a 16-foot (16m) bamboo pole.

Each diver wears a self-contained dive gear consisting of a diving jacket, trousers, shoes, and helmet fixed by four bolts. They were weighed down with 20 Ib (9kg) of lead and were supported by oxygen and liquid food, allowing the divers to sustain themselves while walking at a depth of 16-23 feet for around 6 -10 hours.

Once the target was reached, the diver would stick the bamboo pole under the enemy naval vessel passing overhead, pushing the contact-fuzed explosive into its hull, resulting in a detonation and the killing of the diver in the process.

During the course of the war, two Fukuryu units were created, totalling 1200 out of the proposed 6000 personnel, and only 1000 diving suits were produced. Aside from several training incidents where most of the fatalities were caused by failures in the suit’s breathing apparatus, the Fukuryu never saw action, as Japan surrendered to American Forces in September 1945 after the dropping of the two atomic bombs.

Nevertheless, there were some reports of Japanese suicide swimmers attacking Allied boats in the Palau Islands earlier that year, showing how lethal and possible the concept can be in the event of an Allied invasion.

https://www.warhistoryonline.com/war-articles/11-secret-weapons-developed-by-japan-during-wwii.html

https://www.warhistoryonline.com/world-war-ii/kamikaze-frogmen-fukuryu.html#:~:text=The%2071st%20Arashi%20were%20trained,were%20constructed%2C%20only%20dummy%20ones.

https://en.wikipedia.org/wiki/Fukuryu