Hydrostatical pressure and air-pressure

The fact that the hydrostatical pressure increases with the depth of immersion can be illustrated with the following experiment:
We are fixing a balloon to a long glass tube. Now we are filling the balloon with coloured water and immersing it into a deep container that is filled with water.

You can observe that the balloon is compressed more the deeper it is immersed and therefore the coloured water in the tube rises. With increasing depth of immersion the pressure increases. In the emersion the process is opposite: the water in the tube can go back into the balloon because it is less compressed.

The image shows that the balloon is highly compressed when it is deep in the water.

This principle is used with pressure measurement devices.