The pressure inside the middle ear is maintained equal to the external ear (so to the exterior too) due to a communicating duct between the middle ear and the pharynx called the auditory tube, or Eustachian tube. When someone goes down a mountain the air pressure upon the middle ear increases and it is necessary to do some exercises like fake swallowing to force the opening of the pharyngeal orifice of the auditory tube to equalize the pressure again.