I advise taking a scuba diving course.
You will learn that at every depth you balance the air pressure within your lungs and body with the pressure of the water outside so that the net effect is zero. As you ascend from a depth you must remember to breathe out or there will be too much pressure inside your body, particularly the lungs. You learn to go "Ho Ho Ho" all the way up or you get what is called an "air embolism" and die.
Similarly if you ascend quickly or descend quickly in an airplane, you may need to balance the pressure inside and outside your eardrums.