Underwater diving

Underwater diving is the act of going beneath the water’s surface to engage with the underwater world. The term “diving” alone can be ambiguous, as its meaning shifts with context. Because humans are not naturally suited to the underwater environment—physiologically or anatomically—various tools and technologies have been created to allow deeper and longer dives, as well as to enable different kinds of underwater work.

When diving at ambient pressure, the diver is subject to the full pressure of the surrounding water. This can be done on a single breath (freediving), or with breathing apparatus such as scuba gear or surface-supplied equipment. Saturation diving is a technique used to reduce the risk of decompression sickness after long, deep dives. Atmospheric diving suits can protect the diver from high pressure entirely, while crewed submersibles can reach the deepest parts of the ocean. Remotely operated or robotic machines further reduce human risk.

The underwater environment presents many hazards. Although these risks are largely managed through proper training, skills, equipment, and breathing gases—which vary by dive mode, depth, and purpose—diving remains a relatively dangerous activity. Professional diving is typically regulated by occupational health and safety laws, while recreational diving may have no regulation at all.

Depth limits vary by type of diving: recreational scuba diving is generally restricted to about 40 meters (130 feet), commercial saturation diving to 530 meters (1,740 feet), and atmospheric suit diving to 610 meters (2,000 feet). Diving is also limited to conditions that are not excessively hazardous, though acceptable risk levels differ, and fatal incidents can still occur.

Recreational diving, sometimes called sport diving or subaquatics, is a popular leisure activity. Technical diving is a more challenging form of recreational diving. Professional diving—including commercial, research, or financially motivated work—involves underwater labor. Public safety diving is conducted by law enforcement, fire rescue, and search-and-recovery teams. Military diving includes combat diving, clearance diving, and ship husbandry. Deep sea diving typically uses surface-supplied equipment and often refers to the traditional copper-helmet standard diving dress. Hard hat diving covers any form of helmet diving, including both free-flow and lightweight demand helmets.

The history of breath-hold diving dates back at least to classical times, with evidence of prehistoric hunting and gathering of seafood that may have involved underwater swimming. Technical advances that allow a diver to breathe underwater at ambient pressure are relatively recent, and self-contained breathing systems developed rapidly after the Second World War.

Immersion in water, especially cold water, and exposure to high pressure have physiological effects that limit the depth and duration of ambient pressure dives. Breath-hold endurance is a major limitation, and breathing at high pressure introduces further complications. Technological solutions have greatly extended the depth and duration of human dives, allowing useful underwater work. As of 2009, there was no clear evidence that diving causes long-term neurological dysfunction, except through acute injury.

When a person is immersed, the external hydrostatic pressure of water affects circulation, the renal system, fluid balance, and breathing. This pressure supports the blood’s internal hydrostatic pressure, causing a shift of blood from the limbs into the chest cavity. In hydrated subjects, fluid losses called immersion diuresis soon compensate for this shift. Head-out immersion also causes negative pressure breathing, which contributes to the blood shift. This shift increases the workload on the heart and lungs. Stroke volume is not greatly affected by immersion or pressure changes, but a slowed heart rate—especially from the diving reflex during breath-hold dives—reduces overall cardiac output. Lung volume decreases in an upright position due to the abdomen being pushed upward by hydrostatic pressure, and airway resistance increases as lung volume drops. There appears to be a link between pulmonary edema and increased pulmonary blood flow and pressure, which can cause capillary engorgement, particularly during high-intensity exercise while immersed or submerged.

The diving reflex is a response to immersion that overrides basic homeostatic reflexes. It optimizes respiration by preferentially sending oxygen to the heart and brain, allowing longer periods underwater. This reflex is strong in aquatic mammals like seals, otters, dolphins, and muskrats, and also exists in other mammals, including humans. Diving birds, such as penguins, have a similar reflex. The diving reflex is triggered by chilling the face and holding the breath. It constricts peripheral blood vessels, slows the pulse, redirects blood to vital organs to conserve oxygen, releases red blood cells stored in the spleen, and, in humans, can cause heart rhythm irregularities. Aquatic mammals have evolved physiological adaptations that go beyond this basic reflex.