Biber

Biber was a German midget submarine of the Second World War. Armed with two externally mounted 53-centimetre torpedoes or mines, they were intended to attack coastal shipping. They were among the smallest submarines in the Kriegsmarine.

The Biber was hastily developed to help meet the threat of an Allied invasion of Europe. This resulted in basic technical flaws that, combined with the inadequate training of their operators, meant they never posed a real threat to Allied shipping, despite 324 submarines being delivered. One of the class's few successes was the sinking of the cargo ship Alan-A-Dale.

Several survive in museums, including one in operational condition.

Source: Wikipedia