Submarines monitor the air, land and sea (both above the surface and below it). Submarines are force multipliers: compelling foreign militaries to launch numerous vessels in response to even the threat of a single submarine.
During times of war, submarines are crucial in controlling the seas. They detect and destroy hostile submarines and surface ships, blockade foreign ports and restrict ocean transport. They provide intelligence and underwater protection for surface ships, and are able to detect and lay mines more efficiently than any other navy vessel. Submarines provide a means to land special forces in hostile regions and, if fitted with suitable weapons, are able to strike land targets.
Submarines are also used for a variety of functions in the private sector. The most common are scientific submarines, which explore the world's oceans to further research and to locate sunken ships.
Submarines can also be used for tourism, while unmanned submarines (which are very small and operated remotely from the surface) are used to perform work which is too deep or too dangerous for divers, such as on an oil rig.