Scuba diving

[12] The first commercially practical scuba rebreather was designed and built by the diving engineer Henry Fleuss in 1878, while working for Siebe Gorman in London.

[46] Rebreathers are currently manufactured for the military, technical and recreational scuba markets,[40] but remain less popular, less reliable, more complex to operate, and more expensive than open-circuit equipment.

By always providing the appropriate breathing gas at ambient pressure, demand valve regulators ensure the diver can inhale and exhale naturally and without unnecessary effort, regardless of depth, as and when needed.

Rebreathers are more complex and more expensive than open-circuit scuba, and special training and correct maintenance are required for them to be safely used, due to the larger variety of potential failure modes.

Because rebreathers produce very little bubbles, they do not disturb marine life or make a diver's presence known at the surface; this is useful for underwater photography, and for covert work.

Fins have a large blade area and use the more powerful leg muscles, so are much more efficient for propulsion and maneuvering thrust than arm and hand movements, but require skill to provide fine control.

Several types of fin are available, some of which may be more suited for maneuvering, alternative kick styles, speed, endurance, reduced effort or ruggedness.

The faceplate of the mask is supported by a frame and skirt, which are opaque or translucent, therefore the total field-of-view is significantly reduced and eye-hand coordination must be adjusted.

[66][65] A dry suit also provides thermal insulation to the wearer while immersed in water,[67][68][69][70] and generally protects the entire body except the head, hands, and sometimes the feet.

They add some task loading for the diver as the suit must be inflated and deflated with changes in depth in order to avoid "squeeze" on descent or uncontrolled rapid ascent due to over-buoyancy.

Holding the breath at constant depth for short periods with a normal lung volume is generally harmless, providing there is sufficient ventilation on average to prevent carbon dioxide buildup, and is done as a standard practice by underwater photographers to avoid startling their subjects.

Holding the breath during descent can eventually cause lung squeeze, and may allow the diver to miss warning signs of a gas supply malfunction until it is too late to remedy.

The use of a low-density diluent gas, typically helium, in the breathing mixture can reduce this problem, as well as diluting the narcotic effects of the other gases.

Before starting a dive both the diver and their buddy[note 2] do equipment checks to ensure everything is in good working order, ready for use, and correctly fitted.

[98] Buddy and team diving procedures are intended to ensure that a recreational scuba diver who gets into difficulty underwater is in the presence of a similarly equipped person who will understand the problem and can render assistance.

Divers are trained in procedures for donating and receiving breathing gas from each other in an emergency, and may carry an independent alternative air source if they do not choose to rely on a buddy.

[111] The most common form of physical entrapment is getting snagged on ropes, lines or nets, and the use of a cutting implement is the standard method of dealing with the problem.

Their roles include direct combat, infiltration behind enemy lines, placing mines or using a manned torpedo, bomb disposal or engineering operations.

In some cases diver rescue teams may also be part of a fire department, paramedical service or lifeguard unit, and may be classed as public safety diving.

Where risks are marginally acceptable it may be possible to mitigate the consequences by setting contingency and emergency plans in place, so that injury or damage can be minimised where reasonably practicable.

Diving equipment other than breathing apparatus is usually reliable, but has been known to fail, and loss of buoyancy control or thermal protection can be a major burden which may lead to more serious problems.

[125] Scuba diving in relatively hazardous environments such as caves and wrecks, areas of strong water movement, relatively great depths, with decompression obligations, with equipment that has more complex failure modes, and with gases that are not safe to breathe at all depths of the dive require specialised safety and emergency procedures tailored to the specific hazards, and often specialised equipment.

Drowning occurs as a consequence of preceding problems such as unmanageable stress, cardiac disease, pulmonary barotrauma, unconsciousness from any cause, water aspiration, trauma, environmental hazards, equipment difficulties, inappropriate response to an emergency or failure to manage the gas supply.

Air embolism is also frequently cited as a cause of death, and it, too is the consequence of other factors leading to an uncontrolled and badly managed ascent, possibly aggravated by medical conditions.

No significant associations with risk of decompression sickness or arterial gas embolism were found for asthma, body mass index, cardiovascular disease, diabetes or smoking.

This is a class of emergency where a competent buddy may well be invaluable, as they may be able to disentangle or cut the diver free more easily due to better access to, and view of, the entangling materials.

The physics and physiology knowledge required is fairly basic, and helps the diver to understand the effects of the diving environment so that informed acceptance of the associated risks is possible.

The physiology relates the physics to the effects on the human body, to provide a basic understanding of the causes and risks of barotrauma, decompression sickness, gas toxicity, hypothermia, drowning and sensory variations.

[149] Registration of professional divers trained to these standards may be directly administered by government, as in the case of South Africa, where diver registration is done by the Department of Employment and Labour,[88] or by an approved external agent, as in the case of the Australian Diver Accreditation Scheme (ADAS)[161] The following countries and organisations are members of the European Diving Technology Committee, which publishes minimum standards for commercial diver training and competence accepted by these and some other countries through membership of the IDRCF and IDSA: Austria, Belgium, Croatia, Czech Republic, Denmark, Estonia, Finland, France, Germany, Italy, Latvia, Romania, The Netherlands, Norway, Poland, Portugal, Spain, Slovak republic, Sweden, Switzerland, Turkey, United Kingdom, International Marine Contractors Association (IMCA), International Association of Oil and Gas Producers (IOGP), International Transport Workers' Federation (ITF), International Diving Schools Association (IDSA), European Underwater Federation, and International Diving Regulators and Certifiers Forum (IDRCF).

[173] Jarrod Jablonski and Casey McKinlay completed a traverse from Turner Sink to Wakulla Springs, on 15 December 2007, covering a distance of nearly 36,000 feet (11 km).