Why does a submarine sink and float?

A submarine has two main mechanisms that allow it to sink and return to the surface.

The submarine has dive tanks that can be filled with water to make the ship's weight the same as the water it displaces, or a little more. This causes the boat to sink. Air can be blown into those tanks to reduce the weight of the boat and it rises to the surface.

The second system uses the ship's propulsion and diving planes. These are fins (like wings) that can be tilted slightly up or down. When the ship is moving forward the diving planes are tilted to force the ship up or down. This is most effective when the dive tanks are perfectly balanced so that the weight of the boat exactly matches the weight of water outside.