What you can observe while diving:

Have you already dived quite deep in a swimming pool? And what did you notice?
Did you feel the pressure on your ears?

This is becaus the water pressure increases with depth.

In the middle ear there is an airspace behind the eardrum. If the water pressure increases now while diving deeply, the water pushes more strongly onto the eardrum and the volume of the air in the ear will decrease.

The increasing water pressure causes pain and the feeling of pressure in the ear.