Submarine is a ship that can operate underwater. Submarines have a wide variety and different shapes, ranging from fully automatic or small civilian submersible detectors operated by one or two people and operating for several hours to Russian typhoon class nuclear submarines that can carry hundreds of people and dive continuously for 3-6 months. According to the volume, it can be divided into large (mainly military), medium or small (Pocket submarine, submersible) and underwater automatic machinery.

Most large submarines are cylindrical, and a vertical structure (bridge) is usually set in the middle of the ship, which was called "command tower / command platform" in the early days, with communication, sensors, periscope and control equipment. Today's deep-sea submarines or professional submarines often have no such design.

Since the first World War, submarines have been widely used and played an important role in the navies of many large countries. Their functions include attacking enemy warships or submarines, coastal protection, breaking through blockades, reconnaissance and concealing special forces operations. Submarines are also used for non military purposes, such as marine scientific research, property rescue, exploration and mining, scientific detection, equipment maintenance, search and rescue, submarine cable maintenance, underwater tourism, academic investigation, etc. the super rich even use them as underwater mobile luxury houses.

Submarine is a recognized strategic weapon (especially in disarmament or arms expansion negotiations). Its R & D requires a high and comprehensive industrial capacity. Only a few countries can design and produce it by themselves. In particular, the ballistic missile nuclear submarine is the key pole of the nuclear Trinity. Submarines were also stealth vehicles that existed in earlier times. Submarine noise can be "submerged" in the vast ocean background noise when it is reduced to about 90 dB, which can not be detected by contemporary sonar.

Source: @pulsar.tattoo