A submarine has tanks inside. These are filled with water to make the sub sink. To make the sub rise, the tanks are emptied, making the sub lighter.Submarines have been used since the 18th century, but the first practical submarine was not Built until just prior to World war I. Today most submarines are in military service, but future plans call for the use of submarine oil tankers. Submarines submerge and surface by filling or emptying their sea water tanks, but they also control their depth with a horizontal rudder.