http://swissbaync5d7ykaz7dh7v4qjrb4gmen5aj3bogxrcgbb43ij34bjuyd.onion/pdf/Books/Survival/Medicine/sectionf.htm
Whichever method is used, it must be done frequently on descent because at some point
no maneuver will work; this is the situation when the pressure keeping the tube shut is
too great. If the pressure gradient across the tube (nasopharynx to middle ear) exceeds 90
mm Hg - a gradient reached at only about 4 feet depth - none of the maneuvers will open
the eustachian tube and the diver must ascend to relieve the pressure.