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Mask
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A skirted glass window constructed to provide air space between eyes and water and to permit both eyes to see in the same plane; a regular mask covers eyes and nose only; modern mask skirts are usually made of silicone rather than the older rubber ones.
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Mask Squeeze
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A painful condition when the air inside the mask is compressed by the external pressure creating suction on the face and eyes; can be alleviated by exhaling from the nose; can cause permanent eye damage if not equalized.
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Mask squeeze
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Occurs in rapid descents where the diver neglects to equalize his/her mask. The increase pressure causes tissues around the eyes to swell.
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Mediastinal emphysema
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Air from an over expanding lung escapes into the center of the chest. This puts pressure on the heart and major blood vessels, interfering with circulation. Symptoms are shortness of breath and feeling faint.
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Middle ear
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Air containing space of the ear bordered on one side by the tympanic membrane, which is exposed to any change in ambient pressure. Air pressure in the middle ear space can only be equalized through the Eustachian tube, which controls the middle ear to the back of the nose.
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Mixed gas
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Any non-air mixture (e.g., nitrox), although some authors use the term only for mixes that contain a gas in addition to (or in place of) nitrogen (e.g., helium).
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MOD/ODL
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maximum operating depth/oxygen depth limit. The deepest that a diver can safely go using a particular gas mixture. For example, the MOD for EAN32 (32 per- cent oxygen) is 132 fsw (40 m).
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Multilevel diving
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Spending a period of time at several different depth on a single dive.
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