During World War II, the Italian "Maiale" was used to transport frogmen into a harbor or anchorage occupied by enemy ships. When close to an intended target, the frogmen would exit the Maiale, place a mine on the ship's hull, set a timer, return to their mini-sub, and move away from the target, clear of the explosion. Maiale class submarines were used with great effectiveness against British ships anchored in Gibraltar.

During World War II, the Japanese "Type A" two-man mini-sub was carried "piggy back" by a mother ship, usually another submarine, to its area of operation. Type A submarines were used during the attack on Pearl Harbor, Hawaii and throughout the war, but were generally unsuccessful.

The SS X-1 was a non-commissioned experimental submarine designed to both defend and penetrate shallow waters and enemy harbors where large conventional submarines can not operate.  A unique engineering feature was the closed- loop hydrogen peroxide system that allowed the use of the diesel engine under the water independent from an external air source.  This system provided long term underwater operations in much the same manner as a nuclear powered submarine.  With the adoption of nuclear power in the USS Nautilus, the US Navy abandoned this method of underwater propulsion, though it has been carried on by other navys