How does a submarine submerge, or sink down under the water?
To submerge, it must become heavier than the water under it. A sub is already heavy. Usually, its sides are made of steel. But with the air inside, it still floats. So, to become even heavier, a sub must replace some of its air with water. A submarine has large compartments called ballast tanks that can be filled with either water or air. When it is time to submerge, the ballast tanks are filled with water. As soon as the sub becomes heavy enough, it starts to sink. 3 When it gets to the right depth, a sub needs to stay at that depth in order to travel. To do that, the amount of water and air in the ballast tanks are constantly adjusted.
