USS Parche (SSN-683), a Sturgeon-class submarine. Parche was launched on 13 January 1973. Attributed as being a key resource of the National Underwater Reconnaissance Office, Parche is said to be "the most highly decorated vessel in U.S. history.

After an ocean engineering conversion in 1976, Parche spent decades conducting top-secret special operations and intelligence-gathering missions, including deep-sea cable wiretapping in Soviet waters during the Cold War. 

Shortly after commissioning, Parche was transferred to the Mare Island Naval Shipyard to adapt it for specialized intelligence gathering.

Seabed Skids ("Ski Legs"): Retractable landing skids were fitted to the bottom of the hull. This allowed the submarine to physically rest stationary on the ocean floor directly over Soviet communication cables without drifting or using active propulsion.
 
Auxiliary Thrusters: Extra directional thrusters were installed on the sides of the hull. This gave the ship precise hovering capabilities necessary for delicate deep-sea maneuvers.
 
Divers' Lockout Chamber: A saturation diving chamber—disguised externally to look like a Deep Submergence Rescue Vehicle (DSRV) simulator simulator and nicknamed "The Can"—was installed on the aft escape trunk. This allowed Navy divers to exit the submarine while on the seafloor to physically attach wiretaps to underwater cables.
 
Sacrificing Firepower: To make room for the specialized electronic suites, recording gear, and sonar fish, the submarine had to forfeit most of its internal torpedo storage. Its armament was drastically cut down, leaving it with only four torpedo tubes and four torpedoes exclusively for self-defense.
 
The 100-Foot "Plug": The Navy literally cut the submarine in half and welded a 100-foot (30-meter) hull extension forward of the sail. This massive modification increased the boat's total length to over 401 feet and elevated its submerged displacement to roughly 7,800 tons.Flat-Topped "Missile Deck" Profile: The added section was flat-topped. To an outside observer, it made the fast-attack submarine look superficially like an Ohio-class ballistic missile submarine.
 
Intelligence and Crew Space: This massive extension provided the critical space required to carry advanced signals intelligence (SIGINT) antennas, complex recording electronics, specialized photographic/visual equipment, and additional sonar arrays. It also provided living space for the expanded crew of technicians and operators needed to manage the spy equipment.

Length: 1250mm
Beam: 128.5mm
Max inside dimension for cylinder: 85mm

Total volume: 1234cm3