Submarine
The submarine is a specialized craft that can operate underwater at very high pressures, beyond the capacity of unaided human survival. First put to widespread use during the First World War, submarines are now used by all major navies, in particular the US, Russian and British navies. Civilian submarines and submersibles are used for marine and freshwater science, and for work at depths too great for human divers.
Submarines are always referred to as "boats", except in certain official documents. The term U-Boat is sometimes used in English to designate German submarines. The term comes from the German word for submarine, U-Boot, itself an abbreviation of Unterseeboot ("submarine boat").
Submarines represent one of the widest ranges of capabilities of any vessel. They range from small one- or two-man vessels that can examine the seabed for a few hours to the Russian Typhoon class, which can stay submerged for half a year and carry nuclear missiles capable of destroying several cities.
There are also specialized submarines, such as rescue submarines and tiny, one-man, human-powered submarines designed for inter-university competitions. The diving bell is an older device used for underwater exploration, recovery, construction and rescue.
