Underwater Diving as a human activity, is the practice of descending below the water’s surface to interact with the environment. Immersion in water and exposure to high ambient pressure have physiological effects that limit the depths and duration possible in ambient pressure diving.
Humans are not physiologically and anatomically well adapted to the environmental conditions of diving, and various equipment has been developed to extend the depth and duration of human dives and allow different types of work to be done.
Atmospheric diving suits (ADS) may be used to isolate the diver from the high ambient pressure. Crewed submersibles can extend depth range, and remotely controlled or robotic machines can reduce risk to humans.
The environment exposes the diver to a wide range of hazards, and though the risks are largely controlled by appropriate diving skills, training, types of equipment and breathing gases used depending on the mode, depth, and purpose of diving, it remains a relatively dangerous activity.
Diving activities are restricted to maximum depths of about 40 meters (130 ft) for recreational scuba diving, 530 meters (1,740 ft) for commercial saturation diving, and 610 meters (2,000 ft) wearing atmospheric suits. Diving is also restricted to conditions that are not excessively hazardous, though the level of risk acceptable can vary.
Hard hat diving is any form of diving with a helmet, including the standard copper helmet, and other forms of free-flow and lightweight demand helmets. The history of breath-hold diving goes back at least to classical times, and there is evidence of prehistoric hunting and gathering of seafood that may have involved underwater swimming.
Technical advances allowing the provision of breathing gas to a diver underwater at ambient pressure are recent, and self-contained breathing systems developed at an accelerated rate following the Second World War.