The history of underwater diving starts with freediving as a widespread means of hunting and gathering, both for food and other valuable resources such as pearls and coral. By classical Greek and Roman times commercial applications such as sponge diving and marine salvage were established. Military diving also has a long history, going back at least as far as the Peloponnesian War, with recreational and sporting applications being a recent development. Technological development in ambient pressure diving started with stone weights (skandalopetra) for fast descent. In the 16th and 17th centuries diving bells became functionally useful when a renewable supply of air could be provided to the diver at depth, and progressed to surface-supplied diving helmets—in effect miniature diving bells covering the diver's head and supplied with compressed air by manually operated pumps—which were improved by attaching a waterproof suit to the helmet and in the early 19th century became the standard diving dress. Limitations in the mobility of the surface-supplied systems encouraged the development of both open circuit and closed circuit scuba in the 20th century, which allow the diver a much greater autonomy. These also became popular during World War II for clandestine military operations, and post-war for scientific, search and rescue, media diving, recreational and technical diving. The heavy free-flow surface-supplied copper helmets evolved into lightweight demand helmets, which are more economical with breathing gas, which is particularly important for deeper dives and expensive helium based breathing mixtures, and saturation diving reduced the risks of decompression sickness for deep and long exposures. An alternative approach was the development of the "single atmosphere" or armoured suit, which isolates the diver from the pressure at depth, at the cost of great mechanical complexity and limited dexterity. The technology first became practicable in the middle 20th century. Isolation of the diver from the environment was taken further by the development of remotely operated underwater vehicles in the late 20th century, where the operator controls the ROV from the surface, and autonomous underwater vehicles, which dispense with an operator altogether. All of these modes are still in use and each has a range of applications where it has advantages over the others, though diving bells have largely been relegated to a means of transport for surface-supplied divers. In some cases, combinations are particularly effective, such as the simultaneous use of surface orientated or saturation surface-supplied diving equipment and work or observation class remotely operated vehicles. Although the pathophysiology of decompression sickness is not yet fully understood, decompression practice has reached a stage where the risk is fairly low, and most incidences are successfully treated by therapeutic recompression and hyperbaric oxygen therapy. Mixed breathing gases are routinely used to reduce the effects of the hyperbaric environment on ambient pressure divers. The history of underwater diving can also be understood as a history of changing forms of access to the underwater environment. Technological developments progressively extended human capabilities from brief direct immersion to prolonged, autonomous and remotely mediated forms of underwater exploration. This perspective forms part of the concept of "Sub-Oceanic History", proposed by Michael W. Jung, which examines the historical relationship between access, knowledge production and control in the exploration and use of the ocean interior.
Freediving
Underwater diving was practiced in ancient cultures to gather food and other valuable resources such as pearls and precious coral, and later to reclaim sunken valuables, and to help aid military campaigns. Breathhold diving was the only method available, occasionally using reed snorkels in shallow water, and stone weights for deeper dives. Underwater diving for commercial purposes began in Ancient Greece (500 BC), 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 five minutes to collect sponges. Sponges were not 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 Sea had large amounts of sea trade. As a result, there were many shipwrecks, so divers were often hired to salvage whatever they could from the seabed. Divers would swim down to the wreck and choose the pieces to salvage. Divers were also used in warfare. They could be used for underwater reconnaissance when ships were approaching an enemy harbor, and if underwater defenses were found, the divers would disassemble them if possible. During the Peloponnesian War, divers were used to get past enemy blockades to relay messages and provide supplies to allies or troops that were cut off by the blockade. These divers and swimmers were occasionally used as saboteurs, drilling holes in enemy hulls, cutting ships rigging and mooring lines. In Japan, the Ama divers began to collect pearls about 2,000 years ago. Free-diving was the primary source of income for many Persian Gulf nationals such as Qataris, Emiratis, 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.
Diving bells
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