| Magnetic north |
The north direction shown on a compass. The compass needle is attracted by the earth's internal magnetic field. The true north pole and the magnetic north pole are about 1500km apart. |
| Manifold |
A device that connects two cylinders of a twin set together. It may contain its own 'isolation' valve in the middle. |
| Maritime |
Any activity connected with seafaring. |
| Marlinspike |
A tool for opening the strands of a rope so that they can be spliced. |
| Maximum Depth |
The deepest depth reached during a dive. |
| MCA |
Maritime and Coastguard Agency. |
| MCS |
Marine Conservation Society. |
| MDT |
Maximum Dive Time. The length of time that may be spent at a given depth without being required to make a mandatory decompression stop. Also referred to as No-Stop Time and Non-Decompression Time. |
| Mediastinal emphysema |
The condition that exists when air from an over expansion injury escapes into the chest area near the heart and behind the sternum (breast bone). |
| Meridian |
A line of longitude. |
| MGR |
Mixed Gas Rebreather. |
| Mixed Gases |
Any mixture of gas that deviates from normal (normoxic) atmospheric air. |
| Mooring |
Securing a vessel at a pier or elsewhere by several lines so as to limit its movement. |
| MSA |
The Merchant Shipping Act 1995. The MSA chapter 21 deals with wreck and salvage and is administered by the Receiver of Wreck. |
| Multi Level Dive |
A type of a dive that will not conform to a maximum depth and time profile. Ideally, such dives are conducted with the deepest sections first and then the diver ascends progressively to various shallower depths. |
