Underwater diving

Underwater diving is the practice of going underwater, either with breathing apparatus (scuba diving and surface supplied diving) or by breath-holding (freediving). Atmospheric diving suits may be used to isolate the diver from the effects of high ambient pressure, or the saturation diving technique can be used to reduce the risk of decompression sickness after deep dives.

Diving activities are restricted to relatively shallow depths, as even armored atmospheric diving suits are unable to withstand the pressures of the deeper waters of the world. Diving is also restricted to conditions which are not excessively hazardous, though the level of risk acceptable to the diver can vary considerably. Occasionally divers may dive in liquids other than water.

The term deep sea diving refers to underwater diving, usually with surface supplied equipment, and often refers specifically to the use of standard diving dress with the traditional copper helmet. Hard hat diving is any form of diving with a helmet, including the standard copper helmet, other forms of free-flow helmet and lightweight demand helmets.

Recreational diving is a popular activity (also called sport diving or subaquatics). Technical diving is a form of recreational diving that achieves greater depths or endurance or addresses more challenging conditions than found in normal recreational diving. Professional diving (commercial diving, diving for scientific research purposes or diving for financial gain) takes a range of diving activities to the underwater work site. Public safety diving is the underwater work done by law enforcement, fire rescue, and search & rescue/recovery dive teams, and may be done by professionals or volunteers. Military diving includes combat diving, clearance diving and ship's husbandry diving. Underwater sports is a group of competitive sports using either free-diving, snorkelling or scuba technique, or a combination of these techniques.

Training, types of equipment used and breathing gases used depend on the type of diving.

History

Free-diving

Underwater diving was practised in ancient cultures to reclaim sunken valuables, and to help aid military campaigns. In ancient times free diving without the aid of mechanical devices was the only possibility, with the exception of the occasional use of reeds and leather breathing bladders.^[1] The divers faced the same problems as divers today, such as decompression sickness and blacking out during a breath hold. Because of these dangers, diving in antiquity could be quite deadly.

Underwater diving for commercial, rather than recreational purposes may have begun in Ancient Greece, since both Plato and Homer mention the sponge as being used for bathing. The island of Kalymnos was a main centre of diving for sponges. By using weights (skandalopetra) of as much as 15 kilograms (33 lb) to speed the descent, breath-holding divers would descend to depths up to 30 metres (98 ft) for as much as 5 minutes to collect sponges.^[2] Sponges weren't the only valuable harvest to be found on the sea floor; the harvesting of red coral was also quite popular. A variety of valuable shells or fish could be harvested in this way creating a demand for divers to harvest the treasures of the sea, which could also include the sunken riches of other seafarers.

The Mediterranean had large amounts of sea base trade. As a result, there were many shipwrecks, so divers were often hired to salvage whatever they could from the seabed.^[3] Divers would swim down to the wreck and choose the most valuable pieces to salvage. These salvage divers faced many dangers on the job, and as a result, laws, such as the Lex Rhodia, were enacted that awarded a large percentage of the salvage to the divers; in wrecks deeper than 50 feet, divers received one third of the salvage and in wrecks deeper than 90 feet they received half.

Divers were also used in warfare. Defenses against sea vessels were often created, such as underwater barricades aimed at sinking enemy ships. As the barricades were hidden under the water, divers were often used to scout out the sea bed when ships were approaching an enemy harbor. Once these barricades were found it was divers who were used to disassemble them, if possible.^[4] During the Peloponnesian War, divers were used to get past enemy blockades to relay messages as well as supplies to allies or troops that were cut off by the blockade.^[5] On top of all that these ancient frogmen were used as saboteurs, drilling holes in enemy hulls, cutting ships rigging and mooring. Free-diving was the primary source of income for many Gulf nationals such as Qataris, Emiratis, and Bahrainis and Kuwaitis. As a result, Qatari, Emirati and Bahraini heritage promoters have popularized recreational and serious events associated with freediving, underwater equipment and related activities such as snorkeling.^[6]

Diving bell

Diving bells were developed in the 16th and 17th century as the first significant mechanical aid to underwater diving. They were rigid chambers lowered into the water and ballasted so as to remain upright in the water and to be negatively buoyant so that it sinks even when full of air.

The first basic diving bell was probably constructed in 1535, by Guglielmo de Lorena. In 1616, Franz Kessler built an improved diving bell.^[7][8]

Bells were often used for salvage work. In 1658, Albrecht von Treileben was contracted by King Gustavus Adolphus of Sweden to salvage the warship Vasa, which sank in Stockholm harbor on its maiden voyage in 1628. Between 1663 and 1665 von Treileben's divers were successful in raising most of the cannon, working from a diving bell.^[9] In 1687, Sir William Phipps used an inverted container to recover £200,000-worth of treasure from a Spanish ship sunk off the coast of San Domingo.

In 1691, Dr. Edmond Halley completed plans for a greatly improved diving bell, capable of remaining submerged for extended periods of time, and fitted with a window for the purpose of undersea exploration. The atmosphere was replenished by way of weighted barrels of air sent down from the surface.^[10] In a demonstration, Halley and five companions dived to 60 feet (18 m) in the River Thames, and remained there for over an hour and a half. Improvements made to it over time, extended his underwater exposure time to over 4 hours.^[11][12]

In 1775, Charles Spalding, an Edinburgh confectioner, improved on Dr. Halley's design by adding a system of balance-weights to ease the raising and lowering of the bell, along with a series of ropes for signaling to the surface crew.^[13]

In 1689, Denis Papin had suggested that the pressure and fresh air inside a diving bell could be maintained by a force pump or bellows. His idea was implemented exactly 100 years later by the engineer John Smeaton who built the first workable diving air pump in 1789.^[7][8]

Diving dress

The next advance in diving technology, was the first diving dress designs in the early 18th century. Two English inventors developed the first pressure-proof diving suits in the 1710s. John Lethbridge built a completely enclosed suit to aid in salvage work. It consisted of a pressure-proof air-filled barrel with a glass viewing hole and two watertight enclosed sleeves.^[14] This suit gave the diver more maneouverability to accomplish useful underwater salvage work.

After testing this machine in his garden pond (specially built for the purpose) Lethbridge dived on a number of wrecks: four English men-of-war, one East Indiaman, two Spanish galleons and a number of galleys. He became very wealthy as a result of his salvages. One of his better-known recoveries was on the Dutch Slot ter Hooge, which had sunk off Madeira with over three tons of silver on board.^[15]

At the same time, Andrew Becker created a leather-covered diving suit with a windowed helmet. The suit used a system of tubes for inhaling and exhaling, and Becker demonstrated his suit in the River Thames, London, during which he remained submerged for an hour. These suits were still of limited use as there was still no practical system for replenishing the oxygen supply from the surface during the dive.^[16]

Surface-supplied diving dress

The first successful diving helmets were produced by the brothers Charles and John Deane in the 1820s.^[17] Inspired by a fire accident he witnessed in a stable in England,^[18] he designed and patented a "Smoke Helmet" to be used by firemen in smoke-filled areas in 1823. The apparatus comprised a copper helmet with an attached flexible collar and garment. A long leather hose attached to the rear of the helmet was to be used to supply air - the original concept being that it would be pumped using a double bellows. A short pipe allowed breathed air to escape. The garment was constructed from leather or airtight cloth, secured by straps.

The brothers had insufficient funds to build the equipment themselves, so they sold the patent to their employer, Edward Barnard. It was not until 1827 that the first smoke helmets were built, by German-born British engineer Augustus Siebe. In 1828 they decided to find another application for their device and converted it into a diving helmet. They marketed the helmet with a loosely attached "diving suit" so that a diver could perform salvage work but only in a full vertical position, otherwise water entered the suit.

In 1829 the Deane brothers sailed from Whitstable for trials of their new underwater apparatus, establishing the diving industry in the town. In 1834 Charles used his diving helmet and suit in a successful attempt on the wreck of Royal George at Spithead, during which he recovered 28 of the ship's cannon. In 1836, John Deane recovered from the discovered Mary Rose shipwreck timbers, guns, longbows, and other items.

By 1836 the Deane brothers had produced the world's first diving manual, Method of Using Deane's Patent Diving Apparatus which explained in detail the workings of the apparatus and pump, plus safety precautions.

In the 1830s the Deane brothers asked Siebe to apply his skill to improve their underwater helmet design.^[19] Expanding on improvements already made by another engineer, George Edwards, Siebe produced his own design; a helmet fitted to a full length watertight canvas diving suit. The real success of the equipment was a valve in the helmet.

Siebe introduced various modifications on his diving dress design to accommodate the requirements of the salvage team on the wreck of the HMS Royal George, including making the helmet be detachable from the corset; his improved design gave rise to the typical standard diving dress which revolutionised underwater civil engineering, underwater salvage, commercial diving and naval diving.^[19]

Development of modern diving operations

Royal George, a 100-gun first-rate ship of the line of the Royal Navy, sank undergoing routine maintenance work in 1782. The Deane brothers were commissioned to perform salvage work on the wreck. Using their new air-pumped diving helmets, they managed to recover about two dozen cannons.

Following on from this success, Colonel of the Royal Engineers Charles Pasley commenced large-scale salvage operations in 1839. His plan was to break up the wreck of Royal George with gunpowder charges and then salvage as much as possible using divers.

Pasley's diving salvage operation set many diving milestones, including the first recorded use of the buddy system in diving, when he gave instructions to his divers to operate in pairs. In addition, the first emergency swimming ascent was made by a diver after his air line became tangled and he had to cut it free. A less fortunate milestone was the first medical account of a diver squeeze suffered by a Private Williams: the early diving helmets used had no non-return valves; this meant that if a hose became severed, the high-pressure air around the diver's head rapidly evacuated the helmet causing tremendous negative pressure that caused extreme and sometimes life-threatening effects. At the British Association for the Advancement of Science meeting in 1842, Sir John Richardson described the diving apparatus and treatment of diver Roderick Cameron following an injury that occurred on 14 October 1841 during the salvage operations.^[20]

Pasley recovered 12 more guns in 1839, 11 more in 1840, and 6 in 1841. In 1842 he recovered only one iron 12-pounder because he ordered the divers to concentrate on removing the hull timbers rather than search for guns. Other items recovered, in 1840, included the surgeon's brass instruments, silk garments of satin weave 'of which the silk was perfect', and pieces of leather; but no woollen clothing.^[21] By 1843 the whole of the keel and the bottom timbers had been raised and the site was declared clear.^[22]

Self-contained air supply equipment

The essential drawback to the equipment pioneered by Deane and Siebe was the requirement for a constant supply of air pumped from the surface. This restricted the movements and range of the diver and was also potentially hazardous as the supply could get cut off for a number of reasons.

Early attempts at creating systems that would allow divers to carry a portable oxygen source did not succeed, as the compression and storage technology was not advanced enough to allow compressed air to be stored in containers at sufficiently high pressures. By the end of the nineteenth century, two basic templates for scuba, (self-contained underwater breathing apparatus), had emerged; open-circuit scuba where the diver's exhaust is vented directly into the water, and closed-circuit scuba where the diver's unused oxygen is filtered from the carbon dioxide and recirculated.

Open-circuit scuba

The first important step in the development of scuba technology was the invention of the demand regulator. In 1864, the French engineers Auguste Denayrouze and Benoît Rouquayrol designed their "Rouquayrol-Denayrouze diving suit" after adapting the pressure regulator mechanism for underwater use. Their suit was the first to supply 'air-on-demand' to the user by adjusting the flow of air from the tank according to the diver’s requirements. However, the system still had to use surface supply, as the cylinders of the 1860s were not able to withstand the necessary high pressures.

The first open-circuit scuba system was devised in 1925 by Yves Le Prieur in France. Inspired by the simple apparatus of Maurice Fernez and the freedom it allowed the diver, he immediately conceived an idea to make it free of the tube to the surface pump by using Michelin cylinders as the air supply, containing three litres of air compressed to 150 kg/cm2. The "Fernez-Le Prieur" diving apparatus was demonstrated at the swimming pool of Tourelles in Paris in 1926. The unit consisted of a cylinder of compressed air carried on the back of the diver, connected to a pressure regulator designed by Le Prieur adjusted manually by the diver, with two gauges, one for tank pressure and one for output (supply) pressure. Air was supplied continually to the mouthpiece and ejected through a short exhaust pipe fitted with a valve as in the Fernez design. Le Prieur had invented the self-contained underwater breathing apparatus – scuba.^[23] However, the lack of a demand regulator and the low endurance of the apparatus, limited the practical use of LePrieur’s device.

In 1942, during the German occupation of France, Jacques-Yves Cousteau and Émile Gagnan designed the first successful and safe open-circuit scuba, known as the Aqua-Lung. Their system combined an improved demand regulator with high-pressure air tanks. Émile Gagnan, an engineer employed by the Air Liquide company, miniaturized and adapted the regulator to use with gas generators, in response to constant fuel shortage that was a consequence of German requisitioning. Gagnan's boss, Henri Melchior, knew that his son-in-law Jacques-Yves Cousteau was looking for an automatic demand regulator to increase the useful period of the underwater breathing apparatus invented by Commander le Prieur,^[24] so he introduced Cousteau to Gagnan in December 1942. On Cousteau's initiative, the Gagnan's regulator was adapted to diving, and the new Cousteau-Gagnan patent was registered some weeks later in 1943.^[25] After the war, in 1946, both men founded La Spirotechnique (as a division of Air Liquide) in order to mass-produce and sell their invention, this time under a new 1945 patent, and known as CG45 ("C" for Cousteau, "G" for Gagnan and "45" for 1945). This same CG45 regulator, produced for more than ten years and commercialized in France as of 1946, was the first to actually be called the "Aqua-Lung".

Closed-circuit scuba

The alternative concept, developed in roughly the same time frame was the closed-circuit scuba. The body consumes and metabolises a small fraction of the inhaled oxygen - the situation is even more wasteful of oxygen when the breathing gas is compressed as it is in scuba systems underwater. The rebreather therefore recycles the used oxygen, while constantly replenishing it from the supply so that the oxygen level does not get depleted. The apparatus also has to chemically remove the exhaled carbon dioxide, as a buildup of CO₂ levels would result in respiratory distress and hypercapnia.

The first commercially practical closed-circuit scuba was designed and built by the diving engineer Henry Fleuss in 1878, while working for Siebe Gorman in London.^[7][26] His self-contained breathing apparatus consisted of a rubber mask connected to a breathing bag, with (estimated) 50-60% O₂ supplied from a copper tank and CO₂ scrubbed by rope yarn soaked in a solution of caustic potash; the system giving a duration of about three hours.^[7][27] Fleuss tested his device in 1879 by spending an hour submerged in a water tank, then one week later by diving to a depth of 5.5m in open water, upon which occasion he was slightly injured when his assistants abruptly pulled him to the surface.

His apparatus was first used under operational conditions in 1880 by the lead diver on the Severn Tunnel construction project, who was able to travel 1000 feet in the darkness to close several submerged sluice doors in the tunnel; this had defeated the best efforts of hard hat divers due to the danger of their air supply hoses becoming fouled on submerged debris, and the strong water currents in the workings.^[7]

Fleuss continually improved his apparatus, adding a demand regulator and tanks capable of holding greater amounts of oxygen at higher pressure. Sir Robert Davis, head of Siebe Gorman, perfected the oxygen rebreather in 1910^[7][28] with his invention of the Davis Submerged Escape Apparatus, the first rebreather to be made in quantity. While intended primarily as an emergency escape apparatus for submarine crews, it was soon also used for diving, being a handy shallow water diving apparatus with a thirty-minute endurance,^[28] and as an industrial breathing set.

The rig comprised a rubber breathing/buoyancy bag containing a canister of barium hydroxide to scrub exhaled CO₂ and, in a pocket at the lower end of the bag, a steel pressure cylinder holding approximately 56 litres of oxygen at a pressure of 120 bar. The cylinder was equipped with a control valve and was connected to the breathing bag. Opening the cylinder's valve admitted oxygen to the bag and charged it to the pressure of the surrounding water. The rig also included an emergency buoyancy bag on the front of to help keep the wearer afloat. The DSEA was adopted by the Royal Navy after further development by Davis in 1927.^[29]

In 1912 the German firm Dräger developed their own version of standard diving dress without an umbilical. The air supply also came from a rebreather.

Scuba systems during World War II

In the 1930s, Italian sport spearfishers began to use the Davis rebreather; Italian manufacturers received a license from the English patent holders to produce it. This practice soon came to the attention of the Italian Navy, which developed its frogman unit Decima Flottiglia MAS and was used effectively in World War II.^[27]

During the Second World War, captured Italian frogmen's rebreathers influenced improved designs for British rebreathers.^[27] Many British frogmen's breathing sets used aircrew breathing oxygen cylinders salvaged from shot-down German Luftwaffe aircraft. The earliest of these breathing sets may have been modified Davis Submerged Escape Apparatus; their fullface masks were the type intended for the Siebe Gorman Salvus, but in later operations different designs were used, leading to a fullface mask with one big face window, at first oval and later rectangular (mostly flat, but the sides curved back to allow better vision sideways). Early British frogman's rebreathers had rectangular counterlungs on the chest like Italian frogman's rebreathers, but later designs had a square recess in the top of the counterlung so it could extend further up toward the shoulders. In front they had a rubber collar that was clamped around the absorbent canister.^[27] Some British armed forces divers used bulky thick diving suits called Sladen suits; one version of it had a flip-up single faceplate for both eyes to let the user get binoculars to his eyes when on the surface.

Atmospheric diving suits

The atmospheric diving suit is a small one-man submersible of anthropomorphic form with elaborate pressure joints to allow articulation while maintaining an internal pressure of one atmosphere. Although various atmospheric suits were developed during the Victorian era, none of these suits were able to overcome the basic design problem of constructing a joint which would remain flexible and watertight at depth without seizing up under pressure.

Pioneering diving engineer, Joseph Salim Peress, invented the first truly usable atmospheric diving suit, the Tritonia, in 1932 and was later involved in the construction of the famous JIM suit. Having a natural talent for engineering design, he challenged himself to construct an ADS that would keep divers dry and at atmospheric pressure, even at great depth. By 1929 he solved the weight problem by using cast magnesium instead of steel. He also managed to improve the design of the suit's joints by using a trapped cushion of oil to keep the surfaces moving smoothly. In 1930, Peress revealed the Tritonia suit.^[30] By May it had completed trials and was publicly demonstrated in a tank at Byfleet. In September Peress' assistant Jim Jarret dived in the suit to a depth of 123 m (404 ft) in Loch Ness. The suit performed perfectly, the joints proving resistant to pressure and moving freely even at depth.

The Tritonia suit was upgraded into the first JIM suit, completed in November 1971. This suit underwent trials aboard HMS Reclaim in early 1972, and in 1976, the JIM suit set a record for the longest working dive below 490 feet (150 m), lasting five hours and 59 minutes at a depth of 905 feet (276 m). The first JIM suits were constructed from cast magnesium for its high strength-to-weight ratio and weighed approximately 1,100 pounds (498.95 kg) in air including the diver.

The Exosuit is a new, state-of-the-art atmospheric diving suite that will be first used in 2014 at the Bluewater and Antikythera underwater research expeditions. The Exosuit was developed by Phil Nuytten and the first operational suit is owned and operated by J.F. White Contracting, which is making it available for research projects.^[31]

Physiological discoveries

By the late 19th century, as salvage operations became deeper and longer, an unexplained malady began afflicting the divers; they would suffer breathing difficulties, dizziness, joint pain and paralysis, sometimes leading to death. The problem was already well known among workers building tunnels and bridge footings operating under pressure in caissons and was initially called 'caisson disease' but later the 'bends' because the joint pain typically caused the sufferer to stoop. Early reports of the disease had been made at the time of Pasley's salvage operation, but scientists were still ignorant of its causes.

French physiologist Paul Bert was the first to understand it as decompression sickness. His classical work, La Pression barometrique (1878), was a comprehensive investigation into the physiological effects of air-pressure, both above and below the normal.^[32] He determined that inhaling pressurized air caused the nitrogen to dissolve into the bloodstream; rapid depressurization would then release the nitrogen into its natural gaseous state, forming bubbles that could block the blood circulation and potentially cause paralysis or death. Central nervous system oxygen toxicity was also first described in this publication and is sometimes referred to as the "Paul Bert effect".^[32][33]

John Scott Haldane designed a decompression chamber in 1907 to help make deep-sea divers safer and he produced the first decompression tables for the Royal Navy after extensive experiments with animals.^[19][34][35] These tables established a method of decompression in stages - it remains the basis for decompression methods to this day. Following Haldane's recommendation, the maximum safe operating depth for divers was extended to 200 feet.^[36]

Methods of underwater diving

Diving without breathing apparatus

Underwater diving without breathing apparatus can be loosely categorized as underwater swimming, snorkeling and free diving. These categories overlap considerably. Several competitive underwater sports are practiced without breathing apparatus.

The ability to dive and swim underwater while holding one's breath can be a useful emergency skill, and is an important part of water sport and navy safety training. More generally, entering water from a height is an enjoyable leisure activity, as is underwater swimming without breathing apparatus.

Free diving does not involve the use of external breathing devices, but relies on a diver's ability to hold his or her breath until resurfacing. It includes a range of activities from simple breath-hold diving to competitive apnea dives. Fins and a diving mask are often used in free diving to improve vision and provide more efficient propulsion.

The use of a short breathing tube known as a snorkel allows the diver to breathe with the face remaining immersed, while at the surface. This may also be used when no diving is intended, and the snorkeler remains at the surface.

Scuba diving

Scuba diving is diving with a self-contained underwater breathing apparatus, which is completely independent of surface supply, and provides the diver with the advantages of mobility and horizontal range far beyond what is possible when supplied from the surface by the umbilical hoses of surface-supplied diving equipment (SSDE).

Scuba divers engaged on armed forces covert operations may be referred to as frogmen. This tradition started with the World War II Italian navy combat divers of Decima Flottiglia MAS, the Uomini Rana, named for the frog kick style of underwater swimming used at the time.

Open circuit

Open circuit scuba systems discharge the breathing gas into the environment as it is exhaled, and consist of one or more diving cylinders containing breathing gas at high pressure which is supplied to the diver through a diving regulator, and may include additional cylinders for decompression gas or emergency breathing gas.

Rebreathers

Closed-circuit or semi-closed circuit breathing systems allow recycling of exhaled gases. This reduces the volume of gas used, so that a smaller cylinder, or cylinders, than open circuit scuba may be used for the equivalent dive duration, and giving the ability to spend far more time underwater compared to open circuit for the same gas consumption. Rebreathers also produce far less bubble volume and less noise than scuba, which makes them attractive to military, scientific and media divers.

Surface supplied diving

An alternative to self-contained breathing systems is to supply breathing gases from the surface. A diver's umbilical, or airline hose, from the surface provides gas, communications and a safety line, with options for a hot water hose for heating, a video cable and gas reclaim line.

Surface oriented diving

Surface oriented, or bounce diving, is how commercial divers refer to diving operations where the diver starts and finishes the diving operation at atmospheric pressure. The alternative is saturation diving.

The diver may be deployed directly, often from a diving support vessel or indirectly via a diving bell. Surface-supplied divers almost always wear diving helmets or full face diving masks. The bottom mix can be air or mixed gas, the decompression mix nitrox or pure oxygen.^[37] Decompression procedures include in-water decompression or surface decompression in a deck chamber.

A wet bell with a gas filled dome provides more comfort and control than a stage and allows for longer time in water. Wet bells are used for air and mixed gas, and divers can decompress on oxygen at 12 m.^[37]

Small closed bell systems have been designed that can be easily mobilized, and include a two-man bell, a handling frame and a chamber for decompression after transfer under pressure (TUP). Divers can breathe air or mixed gas at the bottom but are usually recovered with the chamber filled with air. They decompress on oxygen supplied through built in breathing systems (BIBS) towards the end of the decompression. Small bell systems support bounce diving down to 120 m and for bottom times up to 2 hours.^[37]

An alternative to scuba diving, called "SNUBA" or "hooka" diving, has the diver supplied via an airline from a small cylinder or compressor at the surface. It is used for light work such as hull cleaning and archaeological surveys, for shellfish harvesting, and as a shallow water tourist activity for those who are not scuba-certified.

Saturation diving

Saturation diving lets professional divers live and work under pressure for days or weeks at a time. This type of diving allows greater economy of work and enhanced safety. After working in the water, divers rest and live in a dry pressurized underwater habitat on the bottom or a saturation life support system of pressure chambers on the deck of a diving support vessel, oil platform or other floating work station. In either case, they stay at a similar pressure to the work depth. They may be transferred in a closed diving bell, also known as a personnel transfer capsule. Decompression at the end of the dive may take many days, but since it is done only once for a long period of exposure, rather than after each of many shorter exposures, the overall risk of decompression injury to the diver and the total time spent decompressing are reduced.

Dive sites

The common term for a place at which one may dive is a dive site. These are restricted by accessibility and risk, but may include water and occasionally other liquids. Most underwater diving is done in the shallower coastal parts of the oceans, and inland bodies of fresh water, including lakes, dams, quarries, rivers, springs, flooded caves, reservoirs, tanks, swimming pools, and canals, but may also be done in large bore ducting and sewers, power station cooling systems, cargo and ballast tanks of ships, and liquid-filled industrial equipment. Diving in liquids other than water may present special problems due to density, viscosity and chemical compatibility of diving equipment, as well as possible environmental hazards to the diving team.^[38]

As a general rule, professional diving is done where the work is to be done, and recreational diving is done where conditions are suitable. As a consequence, there are many recorded and publicized recreational dive sites which are known for their convenience, points of interest, and frequently favourable conditions. Diver training facilities for both professional and recreational divers will generally use a small range of dive sites which are familiar, convenient and where conditions are predictable and the risk is relatively low.^[39]

Recreational diver service organisations may provide websites or brochures listing the sites to which they provide access, and popular dive sites in many parts of the world have been described in magazines and books, in a widely varying range of detail and accuracy. There are also travel and other specialist websites which provide the recreational diver with facilities to describe dive sites, either as blogs or as co-operative travel guides^[40]

Reasons for diving

Diving may be done for a number of reasons, both personal and professional.

Recreational diving, is purely for enjoyment and has a number of distinct technical disciplines to increase interest underwater, such as cave diving, wreck diving, ice diving and deep diving.

Underwater sport is also done for enjoyment and includes specific sports such as aquathlon (i.e. underwater wrestling), finswimming, free-diving, spearfishing, sport diving, underwater football, underwater hockey, underwater ice hockey, underwater orienteering, underwater photography, underwater rugby, underwater target shooting and underwater videography. Many of these underwater sports can be enjoyed simply for exercise and the associated health benefits, or for true recreation, or indeed for competition at varying levels.

Divers may be employed professionally to perform tasks underwater.

Commercial divers are employed to perform tasks related to industries involving underwater work, including civil engineering tasks such as in oil exploration, offshore construction dam maintenance and harbour works. Commercial divers may also be employed to perform tasks specifically related to marine activities, such as naval diving, including the repair and inspection of boats and ships, salvage of wrecks or aquaculture.

There are a fair number of divers who work, full or part-time, in the recreational diving community as instructors, assistant instructors, divemasters and dive guides. In some jurisdictions the professional nature, with particular reference to responsibility for health and safety of the clients, of recreational diver instruction, dive leadership for reward and dive guiding is recognised by national legislation.^[41][42]

Other specialist areas of diving include military diving, with a long history of military frogmen in various roles. They can perform roles including direct combat, infiltration behind enemy lines, placing mines, bomb disposal or engineering operations.

In civilian operations, many police forces operate police diving teams to perform search and recovery or search and rescue operations and to assist with the detection of crime which may involve bodies of water. 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.

Lastly, there are professional divers involved with the water itself, such as underwater photography or underwater film makers, who set out to document the underwater world, or scientific diving, including marine biology, geology, hydrology, oceanography and underwater archaeology. In North Carolina wreck divers regularly visit the WWII shipwrecks to dive with the Sand Tiger sharks that make the wrecks their home.^[43]

The choice between scuba and surface supplied diving equipment is based on both legal and logistical constraints. Where the diver requires mobility and a large range of movement, scuba is usually the choice if safety and legal constraints allow. Higher risk work, particularly commercial diving, may be restricted to surface supplied equipment by legislation and codes of practice.

Reasons for diving may include:

Diving activities	Classification	Scuba or Surface Supplied Diving Equipment
aquarium maintenance in large public aquariums	commercial, scientific	Scuba, SSDE
boat and ship inspection, cleaning and maintenance	commercial, naval	SSDE, occasionally scuba
cave diving	technical, recreational, scientific	Scuba, occasionally SSDE
civil engineering in harbours, water supply, and drainage systems	commercial	Almost exclusively SSDE
crude oil industry and other offshore construction and maintenance	commercial	Almost exclusively SSDE
demolition and salvage of ship wrecks	commercial, naval	SSDE, sometimes scuba
professional diver training	professional	SSDE or scuba as appropriate
recreational diver training	professional, recreational	Scuba
fish farm maintenance	commercial	Scuba, SSDE
fishing, e.g. for abalones, crabs, lobsters, pearls, scallops, sea crayfish, sponges	commercial	Scuba, SSDE
frogman, manned torpedo	military	Scuba
harbour clearance and maintenance	commercial, military	Almost exclusively SSDE
media diving: making television programs, etc.	professional	Scuba, occasionally SSDE
mine clearance and bomb disposal, disposing of unexploded ordnance	military, naval	Scuba, occasionally SSDE
pleasure, leisure, sport	recreational	Almost exclusively scuba
policing/security: diving to investigate or arrest unauthorized divers	police diving, military, naval	Scuba
search and recovery diving	commercial, public safety, police diving	Scuba, SSDE
search and rescue diving	police, naval, public service	Scuba, occasionally SSDE
spear fishing	professional (occasionally), recreational, competitive	Breathhold
stealthy infiltration	military	Scuba
surveys and mapping	scientific, recreational	Scuba, SSDE
scientific diving (marine biology, oceanography, hydrology, geology, palaeontology, diving physiology and medicine)	scientific	Scuba, occasionally SSDE
underwater archaeology (shipwrecks; harbors, and buildings)	scientific, recreational	Scuba, SSDE
underwater hockey	recreational, competitive	Snorkel, breathhold
underwater inspections and surveys	commercial, military	SSDE, sometimes scuba
underwater photography	professional, recreational	Scuba, SSDE
underwater sport	recreational	Snorkel, breathhold and Scuba
underwater tour guiding	professional, recreational	Scuba
underwater tourism	recreational	Scuba, occasionally Snuba
underwater welding	commercial	Almost exclusively SSDE

Diver training

Underwater diver training is normally given by a qualified instructor who is a member of one of many diving training agencies or is registered with a government agency.

Basic diver training entails the learning of skills required for the safe conduct of activities in an underwater environment, and includes procedures and skills for the use of diving equipment, safety, emergency self-help and rescue procedures, dive planning, and use of dive tables.

Some of the skills which an entry level diver will normally learn include:

• Ear equalization and equalisation of other air spaces.
• Underwater breathing – the skill of breathing through the apparatus.
• Mask clearing – the skill of clearing water from the mask.
• Air sharing – assisting another diver by providing air from one's own supply, or receiving air supplied by another diver.
• Emergency ascents - how to return to the surface without injury in the event of a breathing supply interruption.
• Use of bailout systems (professional divers)
• Buoyancy control – neutral buoyancy allows the diver to move about underwater comfortably.
• Diving signals – used to communicate underwater. Professional divers will also learn other methods of communication.

Some knowledge of physiology and the physics of diving is considered necessary by most diver certification agencies, as the diving environment is alien and relatively hostile to humans. 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 physics mostly relates to gases under pressure, buoyancy, heat loss, and light underwater. 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.

More advanced training often involves first aid and rescue skills, skills related to specialized diving equipment, and underwater work skills.

Diving hazards

Divers face specific physical and health risks when they go underwater with scuba or other diving equipment, or use high pressure breathing gas. The hazards can be listed under several categories:

• The aquatic environment
• Use of breathing equipment in an underwater environment
• Exposure to a pressurised environment and pressure changes, which includes:

• Pressure changes during descent
• Pressure changes during ascent
• Breathing gases at high ambient pressure

• The specific diving environment
• Pre-existing physiological and psychological conditions in the diver
• Diver behaviour and competence
• Failure of diving equipment other than breathing apparatus
• Hazards of the dive task and special equipment
• Hazards related to access to and egress from the water.

The presence of a combination of several hazards simultaneously is common in diving, and the effect is generally increased risk to the diver, particularly where the occurrence of an incident due to one hazard triggers other hazards with a resulting cascade of incidents. Many diving fatalities are the result of a cascade of incidents overwhelming the diver, who should be able to manage any single reasonably foreseeable incident.

The assessed risk of a dive would generally be considered unacceptable if the diver is not expected to cope with any single reasonably foreseeable incident with a significant probability of occurrence during that dive. Precisely where the line is drawn depends on circumstances. Commercial diving operations tend to be less tolerant of risk than recreational, particularly technical divers, who are less constrained by occupational health and safety legislation.

According to a North American 1972 analysis of calendar year 1970 data, diving was, based on man hours, 96 times more dangerous than driving an automobile.^[44] According to a 2000 Japanese study, every hour of recreational diving is 36 to 62 times riskier than automobile driving.^[45]

Consequences of diving hazards range from merely annoying through to rapidly fatal, and are listed in the article on Diving hazards and precautions and discussed in detail in other articles linked from that article.

Other forms of underwater diving

Diving in submersibles

Submarines, submersibles and 'hard' diving suits enable undersea diving to be carried out within a dry environment at normal atmospheric pressure, albeit more remotely. Underwater robots and remotely operated vehicles and also carry out some functions of divers at greater depths and in more dangerous environments.

Diving by other animals

Humans are not the only air-breathing creatures to dive. Marine mammals such as seals, dolphins and whales, dive to feed and catch prey under the sea as do penguins and many seabirds, as well as various reptiles: turtles,^[46] saltwater crocodiles, seasnakes and Marine Iguanas. Many mammals, birds and reptiles also dive in freshwater rivers and lakes.

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  Freediver with monofin, ascending.

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  Recreational scuba diver

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  Standard diving dress

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  Surface supplied diver riding a stage to the underwater workplace.

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  Closed bell used to transfer saturation divers from the workplace to the hyperbaric life support system.

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  US Navy Atmospheric Diving System (ADS)

See also

• List of sports films

References

Further reading

• Cousteau J.Y. (1953) Le Monde du Silence, translated as The Silent World, Hamish Hamilton Ltd., London; ASIN B000QRK890
• Lang M.A. & Brubakk A.O. (eds., 2009) The Future of Diving: 100 Years of Haldane and Beyond, Smithsonian Institution Scholarly Press, Washington DC

External links

Media related to Underwater diving at Wikimedia Commons

• Scuba divers swim among the sharks, Fayetteville Observer

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