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Scuba diving is still evolving, but general classifications have grown to describe various diving activities. These classifications include:

Commercial diving
Military diving
Naval diving
Police diving
Professional diving
Recreational diving
Rescue and recovery diving
Scientific diving
Technical diving
Cave diving
Cavern diving
Deep diving
Ice diving
Wreck diving

Type of diving Classification
aquarium maintenance in large public aquariums commercial, scientific
boat and ship inspection, cleaning and maintenance commercial, naval
cave diving technical, recreational
civil engineering in harbors, water supply, and drainage systems commercial
crude oil industry and other offshore construction and maintenance commercial
demolition and salvage of ship wrecks commercial, naval
diver training for reward professional
fish farm maintenance commercial
fishing, e.g. for abalones, crabs, lobsters, pearls, scallops, sea crayfish, sponges commercial
frogman, manned torpedo military
harbor clearance and maintenance commercial, military
media diving: making television programs, etc. professional
mine clearance and bomb disposal, disposing of unexploded ordnance military, naval
pleasure, leisure, sport recreational
underwater photography professional, recreational
policing diving to investigate or arrest unauthorized divers police, military, naval
search and recovery diving commercial
search and rescue diving police
spear fishing professional (occasionally), recreational
stealthy infiltration military
marine biology scientific, recreational
underwater tourism recreational
underwater archaeology (shipwrecks; harbors, and buildings) scientific, recreational
underwater welding commercial

Breathing underwater
 
The diver down flag - Designates a diver is in the waterFor more information, see Diving regulator.
Water normally contains dissolved oxygen from which fish and other aquatic animals extract all their required oxygen as the water flows past their gills. Humans lack gills and do not otherwise have the capacity to breathe underwater unaided by external devices.

Early diving experimenters quickly discovered it is not enough simply to supply air in order to breathe comfortably underwater. As one descends, in addition to the normal atmospheric pressure, water exerts increasing pressure on the chest and lungs — approximately 1 bar or 14.7 psi for every 33 feet or 10 meters of depth — so the pressure of the inhaled breath must almost exactly counter the surrounding or ambient pressure to inflate the lungs. It generally becomes difficult to breathe through a tube past three feet under the water.

By always providing the breathing gas at ambient pressure, modern demand valve regulators ensure the diver can inhale and exhale naturally and virtually effortlessly, regardless of depth.

Because the diver's nose and eyes are covered by a diving mask; the diver cannot breathe in through the nose, except when wearing a full face diving mask. However, inhaling from a regulator's mouthpiece becomes second nature very quickly.

Now there are two main circuits in the scuba diving breating apparatus!
 Open-circuit
The most commonly used scuba set today is the "single-hose" open circuit 2-stage diving regulator, coupled to a single pressurized gas cylinder, with the first stage on the cylinder and the second stage at the mouthpiece. This arrangement differs from Emile Gagnan's and Jacques Cousteau's original 1942 "twin-hose" design, known as the Aqua-lung, in which the cylinder's pressure was reduced to ambient pressure in one or two or three stages which were all on the cylinder. The "single-hose" system has significant advantages over the original system.

In the "single-hose" two-stage design, the first stage regulator reduces the cylinder pressure of about 200 bar (3000 psi) to an intermediate level of about 10 bar (145 psi) The second stage demand valve regulator, connected via a low pressure hose to the first stage, delivers the breathing gas at the correct ambient pressure to the diver's mouth and lungs. The diver's exhaled gases are exhausted directly to the environment as waste. The first stage typically has at least one outlet delivering breathing gas at unreduced tank pressure. This is connected to the diver's pressure gauge or computer, in order to show how much breathing gas remains.

Nitrogen intoxication is a common ailment ,alebeit somwhat serious that may affect deep sea divers!!
Rebreather
 
An Inspiration electronic fully closed circuit rebreatherMain article: Rebreathers
Less common, but becoming increasingly available, are closed and semi-closed rebreathers. Open-circuit sets vent off all exhaled gases, but rebreathers reprocess each exhaled breath for re-use by removing the carbon dioxide buildup and replacing the oxygen used by the diver. Rebreathers release few or no gas bubbles into the water, and use much less oxygen per hour because exhaled oxygen is recovered; this has advantages for research, military, photography, and other applications. The first modern rebreather was the MK-19 that was developed at S-Tron by Ralph Osterhout that was the first electronic system. Rebreathers are more complex and more expensive than sport open-circuit scuba, and need special training and maintenance to be safely used.