You can try to yawn. A yawn is a reflex consisting of the simultaneous inhalation of air and the stretching of the eardrums, followed by an exhalation of breath. Of course, try to yawn keeping your mouth closed as you most probably will be under water. You can do this by keeping your lips together but lifting your jaw. Then try breathing in and out. Most of the time this helps you to be able to yawn on command. Do this a few times until you feel your ears pop.
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