In 1868 the Kaiser, increasingly-frustrated with Prussia's inability to compete with the Imperial Navy, gave his approval to increase the size and sophistication of Prussia's fleet of submersible craft. The Mark III (pictured above in a rare photograph) was the result.
Although the Mark III could travel further, faster and deeper than any previous submersible, it was never popular amongst Prussian sailors. Constructed predominantly of wood, it was prone to leaking at even moderate depths and soon earned itself the nickname "the water closet". This was to detract somewhat from the Kaiser's objective of establishing a fearsome reputation for his submersible fleet.
Nonetheless, Prussian submersibles began roaming freely throughout the North Sea and North Atlantic Ocean, much to Admiralty Office chagrin, and as Prussian naval engineering techniques developed, subsequent vessels were to become the envy of navies across the world.
Only a handful of images of the Mark III have ever been recorded. A small collection of them is available for viewing here.