Why can a submarine sink to the bottom of the ocean and rise again on command? Here is an experiment to demonstrate the reason for this phenomenon...
1) Plastic bottle ~ like a 2 liter coke bottle
2) Plastic pen cap
3) Small piece of clay ~ or playdoh
4) Water or something to put in the bottle
1) Put water (or whatever) in the bottle.
2) Put the clay on the "arm" of the pen cap.
3) Drop the pen cap in the water so it floats.
4) Put the top on the bottle ~ make sure it is air-tight.
5) Finally...squeeze the sides of the bottle, and watch the cap sink!
Why does this work??
By squeezing the sides of the bottle, you increase the pressure inside the bottle. The added pressure forces water into the pen cap, which adds weight and makes it sink.
The same thing works with a submarine. If a submarine's average density is greater than that of water, it will sink. If it is less, it will float. Since air is less dense than water, if a submarine is full of air, it will float. Just as with the pen cap, water is pumped in and out of ballast tanks to make it sink ("dive" in sub jargon) or float ("on the fly").
Learn more about the principle behind this experiment.