The following units can be upgraded to Submarine:
Submarines are invisible to most units. They can explore rival territory and can carry special units, but are visible (vulnerable) to enemy Destroyers and other submarines, especially Attack Submarines, which gain a +50% strength bonus against regular Submarines.
A submarine is a submersible warship, designed to attack enemy vessels while remaining hidden underwater. Although defensively weak, submarines are feared because of their ability to make a stealthy approach and attack without warning. Often, the presence of a submarine is not detected until the first of the sub's torpedoes strikes its target. The Germans first used submarine warfare extensively during World War I.
Developments after the war, such as sonar, made the submarine even more effective. During World War II, submarines were used extensively by all the world's naval powers. Today's submarines, in addition to their role as fast attack vessels, also carry nuclear weapons that can be fired from underwater positions just off the coast of an enemy country.