Submarine
It was the first submarine combat loss since World War II. In 1982 during the Falklands War, the Argentine cruiser General Belgrano was sunk by the British submarine HMS Conqueror, the first sinking by a nuclear-powered submarine in war. Some weeks later, on 16 June, during the Lebanon War, an unnamed Israeli submarine torpedoed and sank the Lebanese coaster Transit, which was carrying 56 Palestinian refugees to Cyprus, in the belief that the vessel was evacuating anti-Israeli militias. The ship was hit by two torpedoes, managed to run aground but eventually sank. There were 25 dead, including her captain. The Israeli Navy disclosed the incident in November 2018.

Usage: 

Military: Before and during World War II, the primary role of the submarine was anti-surface ship warfare. Submarines would attack either on the surface using deck guns, or submerged using torpedoes. They were particularly effective in sinking Allied transatlantic shipping in both World Wars, and in disrupting Japanese supply routes and naval operations in the Pacific in World War II.
Mine-laying submarines were developed in the early part of the 20th century. The facility was used in both World Wars. Submarines were also used for inserting and removing covert agents and military forces in special operations, for intelligence gathering, and to rescue aircrew during air attacks on islands, where the airmen would be told of safe places to crash-land so the submarines could rescue them. Submarines could carry cargo through hostile waters or act as supply vessels for other submarines.
Submarines could usually locate and attack other submarines only on the surface, although HMS Venturer managed to sink U-864 with a four torpedo spread while both were submerged. The British developed a specialized anti-submarine submarine in WWI, the R class. After WWII, with the development of the homing torpedo, better sonar systems, and nuclear propulsion, submarines also became able to hunt each other effectively.
The development of submarine-launched ballistic missile and submarine-launched cruise missiles gave submarines a substantial and long-ranged ability to attack both land and sea targets with a variety of weapons ranging from cluster bombs to nuclear weapons.
The primary defense of a submarine lies in its ability to remain concealed in the depths of the ocean. Early submarines could be detected by the sound they made. Water is an excellent conductor of sound (much better than air), and submarines can detect and track comparatively noisy surface ships from long distances. Modern submarines are built with an emphasis on stealth. Advanced propeller designs, extensive sound-reducing insulation, and special machinery help a submarine remain as quiet as ambient ocean noise, making them difficult to detect. It takes specialized technology to find and attack modern submarines.

Active sonar uses the reflection of sound emitted from the search equipment to detect submarines. It has been used since WWII by surface ships, submarines and aircraft (via dropped buoys and helicopter "dipping" arrays), but it reveals the emitter's position, and is susceptible to counter-measures.
A concealed military submarine is a real threat, and because of its stealth, can force an enemy navy to waste resources searching large areas of ocean and protecting ships against attack. This advantage was vividly demonstrated in the 1982 Falklands War when the British nuclear-powered submarine HMS Conqueror sank the Argentine cruiser General Belgrano. After the sinking the Argentine Navy recognized that they had no effective defense against submarine attack, and the Argentine surface fleet withdrew to port for the remainder of the war. An Argentine submarine remained at sea, however.

Civilian: Although the majority of the world's submarines are military, there are some civilian submarines, which are used for tourism, exploration, oil and gas platform inspections, and pipeline surveys. Some are also used in illegal activities.
The Submarine Voyage ride opened at Disneyland in 1959, but although it ran under water it was not a true submarine, as it ran on tracks and was open to the atmosphere. The first tourist submarine was Auguste Piccard, which went into service in 1964 at Expo64. By 1997, there were 45 tourist submarines operating around the world. Submarines with a crush depth in the range of 400–500 feet (120–150 m) are operated in several areas worldwide, typically with bottom depths around 100 to 120 feet (30 to 37 m), with a carrying capacity of 50 to 100 passengers.
In a typical operation a surface vessel carries passengers to an offshore operating area and loads them into the submarine. The submarine then visits underwater points of interest such as natural or artificial reef structures. To surface safely without danger of collision the location of the submarine is marked with an air release and movement to the surface is coordinated by an observer in a support craft.
A recent development is the deployment of so-called narco-submarines by South American drug smugglers to evade law enforcement detection. Although they occasionally deploy true submarines, most are self-propelled semi-submersibles, where a portion of the craft remains above water at all times. In September 2011, Colombian authorities seized a 16-meter-long submersible that could hold a crew of 5, costing about $2 million. The vessel belonged to FARC rebels and had the capacity to carry at least 7 tonnes of drugs.

	Civilian submarines

Polar operations: 1903 – Simon Lake submarine Protector surfaced through ice off Newport, Rhode Island.
1930 – USS O-12 operated under ice near Spitsbergen.
1937 – Soviet submarine Krasnogvardeyets operated under ice in the Denmark Strait.
1941–45 – German U-boats operated under ice from the Barents Sea to the Laptev Sea.
1946 – USS Atule used upward-beamed fathometer in Operation Nanook in the Davis Strait.
1946–47 – USS Sennet used under-ice sonar in Operation High Jump in the Antarctic.
1947 – USS Boarfish used upward-beamed echo sounder under pack ice in the Chukchi Sea.
1948 – USS Carp developed techniques for making vertical ascents and descents through polynyas in the Chukchi Sea.
1952 – USS Redfish used an expanded upward-beamed sounder array in the Beaufort Sea.
1957 – USS Nautilus reached 87 degrees north near Spitsbergen.
3 August 1958 – Nautilus used an inertial navigation system to reach the North Pole.
17 March 1959 – USS Skate surfaced through the ice at the north pole.
1960 – USS Sargo transited 900 miles (1,400 km) under ice over the shallow (125 to 180 feet or 38 to 55 metres deep) Bering-Chukchi shelf.
1960 – USS Seadragon transited the Northwest Passage under ice.
1962 – Soviet November-class submarine K-3 Leninsky Komsomol reached the north pole.
1970 – USS Queenfish carried out an extensive undersea mapping survey of the Siberian continental shelf.
1971 – HMS Dreadnought reached the North Pole.
USS Gurnard conducted three Polar Exercises: 1976 (with US actor Charlton Heston aboard); 1984 joint operations with USS Pintado; and 1990 joint exercises with USS Seahorse.
6 May 1986 – USS Ray, USS Archerfish and USS Hawkbill meet and surface together at the Geographic North Pole. First three-submarine surfacing at the Pole.
19 May 1987 – HMS Superb joined USS Billfish and USS Sea Devil at the North Pole.
March 2007 – USS Alexandria participated in the Joint US Navy/Royal Navy Ice Exercise 2007 (ICEX-2007) in the Arctic Ocean with the Trafalgar-class submarine HMS Tireless.
March 2009 – USS Annapolis took part in Ice Exercise 2009 to test submarine operability and war-fighting capability in Arctic conditions.

Technology: 

Buoyancy and trim: All surface ships, as well as surfaced submarines, are in a positively buoyant condition, weighing less than the volume of water they would displace if fully submerged. To submerge hydrostatically, a ship must have negative buoyancy, either by increasing its own weight or decreasing its displacement of water. To control their displacement and weight, submarines have ballast tanks, which can hold varying amounts of water and air.
For general submersion or surfacing, submarines use the main ballast tanks (MBTs), which are ambient pressure tanks, filled with water to submerge or with air to surface. While submerged, MBTs generally remain flooded, which simplifies their design, and on many submarines, these tanks are a section of the space between the light hull and the pressure hull. For more precise control of depth, submarines use smaller depth control tanks (DCTs)—also called hard tanks (due to their ability to withstand higher pressure) or trim tanks. These are variable buoyancy pressure vessels, a type of buoyancy control device. The amount of water in depth control tanks can be adjusted to hydrostatically change depth or to maintain a constant depth as outside conditions (mainly water density) change. Depth control tanks may be located either near the submarine's center of gravity, to minimise the effect on trim, or separated along the length of the hull so they can also be used to adjust static trim by transfer of water between them.