The Kaiten was the underwater equivalent of the kamikaze plane. Kaiten translates literally as 'Return to the sky'.
The concept was simple. Place a mentally stable (important) young man inside a 9 meter torpedo and lock it from the outside. Deploy it from a submarine and let the young man steer the torpedo of death right into the side of a ship thus detonating the 300kg warhead and doing some serious damage.
Imagine that. What a nightmare.