Powered by rechargeable lithium-ion batteries, equipped with state-of-the-art navigation, communication and onboard life sustaining systems the SDV appears as modern day science-fiction; but, these ghost like submersibles have been around since World War II and have served are nations secret wars ever since.
In any type of warfare, secrecy and the element of surprise have always and will always produce the upper hand. When absolute secrecy is required, there is no other element better than water and no better place to get the job done than under it.
The Office of Strategic Services Maritime Units (OSS MU) first introduced combatant submersibles referred to as "submersible canoes." The "Sleeping Beauty" developed by the British and employed by the OSS MU for training was one of the fist and it set the foundation for many advancements to come.
Throughout World War II and beyond the OSS MU continued to design and operate these "Underwater Canoes." Eventually the program was turned over to the Underwater Demolition Teams where they remained.