Most large modern military submarines are powered by nuclear reactors. These submarines often have a system which can get air from the seawater. These two things allow them to stay underwater for long periods of time. Their most important uses are attacking ships or launching missiles. These include cruise missiles and nuclear missiles. There are two main kinds of subs. Attack submarines are small and fast. They attack other subs and surface ships with torpedos. Missile subs are larger and slower. They are built to shoot missiles at far-away targets on land.