A closer look
During the Franco-Prussian War in 1870, Paris was completely surrounded and cut off by enemy forces. Desperate to keep communication lines open, the French built small, rudimentary, un-crewed zinc submarines affectionately called "Boules de Moulins." They stuffed them full of mail and tossed them into the Seine river upstream, hoping the current would carry them secretly into the city under the enemy's noses. Naturally, they almost all got stuck in the mud and failed miserably, but you have to respect the aquatic hustle.
