A midget submarine is a small submarine which is designed for short trips, with a small crew and minimal comforts. By contrast, a full-sized submarine is designed to accommodate crew for weeks or months at a time. Midget submarines tend to have small ranges and they can only remain submerged for short periods, but they can be extremely useful, and they are utilized in a wide range of ways.
The term “midget submarine” is usually used in a military sense, to refer to a small submarine which has been designed for a military purpose such as launching torpedoes, tracking enemy shipping, or spying. By contrast, civilians usually use the term “submersible,” in the sense of a small submarine used for underwater research which can range from searching for shipwrecks to looking at undersea life.
A typical midget submarine accommodates between two and eight people. There are no sleeping quarters, and room to move is often limited, with people taking up stations in the submarine before launch and remaining in position throughout the mission. Many midget submarines lack bathroom facilities and areas to store or prepare food, so they are really only designed for missions of a few hours in duration.
Classically, a midget submarine is launched by a mother ship which carries the submarine close to its target. The ship has facilities to store and work on the midget submarine, and it can potentially remain at sea for an extended period of time, ferrying the midget submarine from location to location as it is needed. Midget submarines may be totally self-contained with their own life support and steering systems, or they may be linked to the mother ship with cables, a practice which is more common with civilian submersibles.
There are some advantages to a midget submarine. It's easier to maneuver and use than a big submarine, and it's also much cheaper to build, maintain, and run. A nation could have a fleet of midget submarines instead of one large sub, thus allowing it to cover more area. A midget submarine is also harder for enemy ships to detect, since it is smaller and quieter than a regular submarine, and this was a big bonus during World War Two, when midget submarines reached some surprising places.
Several examples of vintage military midget submarines can be seen on display in maritime museums, with a number of examples being of German and Japanese construction. Retired submersibles are also sometimes on display at science museums, especially museums which focus on ocean exploration and marine biology.