Submarine is an independent watercraft capable of operations below the surface of the water.
Submarines start their history from the 1620s when Cornelius Drebbel have built a submarine for James I in England following the design of William Bourne. However the military potential of the craft was soon recognized but the first military submarine wasn't built until 1775 by David Bushnell and was called Turtle. It was employed though unsuccessfully during the American Revolutionary War.
Military submarines have become a more functional weapon with the use of a Diesel Electric Propulsion which debuted in 1881 with Fenian Ram, and were used first in Russo-Japan war but much more successfully by Germany during World War I and II.
Today military submarines are powered by nuclear reactors and are capable of prolonged operations and carrying ballistic missiles
But there are also uses in Civil exploration for deep sea exploration as well as for undersea tours.