Air-independent propulsion

Air-Independent Propulsion (AIP), is a term that encompasses technologies which allow a submarine to operate without the need to surface or use a snorkel to access atmospheric oxygen. The term usually excludes the use of nuclear power, and describes augmenting or replacing the diesel-electric propulsion system of non-nuclear vessels. The United States Navy uses the hull classification symbol "SSI" to designate boats powered by AIP, while retaining "SSK" for classic diesel-electric hunter-killers.
See more at Wikipedia.org...