Underwater diving

Underwater diving is the act of going beneath the water’s surface to engage with the underwater world. The word “diving” alone can be ambiguous, as its meaning shifts depending on the situation. Since humans are not naturally built for life underwater, various tools and technologies have been created to allow them to go deeper and stay down longer, enabling a range of underwater tasks.

When diving at ambient pressure, the diver is directly exposed to the surrounding water pressure. This can be done on a single breath (freediving), or with breathing gear like scuba or surface-supplied equipment. For very long or deep dives, saturation diving helps reduce the risk of decompression sickness. Alternatively, an atmospheric diving suit can keep the diver isolated from high pressure, while crewed submersibles can reach the deepest parts of the ocean. Remotely operated or robotic machines can also be used to keep humans out of harm’s way.

The underwater environment presents many hazards, and although proper training, equipment, and breathing gases can manage most risks, diving remains a relatively dangerous activity. Professional diving is usually regulated by workplace safety laws, but recreational diving often has no formal oversight. Recreational scuba diving is generally limited to about 40 meters deep, commercial saturation diving can go to around 530 meters, and atmospheric suits allow dives to roughly 610 meters. Diving is only done when conditions are not overly hazardous, though what counts as acceptable risk varies, and fatal accidents can still happen.

Recreational diving, sometimes called sport diving or subaquatics, is a popular pastime. Technical diving is a more challenging version of recreational diving. Professional diving—whether commercial, research, or for financial gain—involves working underwater. Public safety diving is done by law enforcement, fire rescue, and search-and-recovery teams. Military diving includes combat diving, clearance diving, and ship maintenance. Deep sea diving usually refers to surface-supplied diving, often with traditional copper-helmet gear. Hard hat diving covers any helmet-based diving, including both free-flow and lightweight demand helmets.

Breath-hold diving dates back at least to classical times, with evidence of prehistoric humans swimming underwater to gather seafood. Advances that let divers breathe underwater at ambient pressure are much more recent, and self-contained breathing systems developed rapidly after World War II.

Immersion in water and exposure to cold and high pressure create physiological effects that limit how deep and how long a person can dive. Holding one’s breath is a major limitation, and breathing under high pressure adds further complications. Technology has greatly extended the depth and duration possible for ambient pressure dives, allowing useful work to be done. As of 2009, there was no clear evidence that diving causes long-term brain problems, except through acute injury.

When a person is immersed, the water’s hydrostatic pressure supports the body’s internal blood pressure, causing blood to shift from the limbs into the chest. This triggers fluid loss known as immersion diuresis in hydrated people soon after entering the water. The pressure from head-out immersion also causes negative pressure breathing, which adds to the blood shift. This shift increases the workload on the heart and lungs. Stroke volume stays fairly stable, but a slower heart rate reduces overall cardiac output, especially during the diving reflex in breath-hold diving. Lung volume decreases when upright because hydrostatic pressure pushes the abdomen upward, and airway resistance increases as lung volume shrinks. There appears to be a link between pulmonary edema and increased blood flow and pressure in the lungs, which can cause capillary engorgement, particularly during intense exercise while immersed.

The diving reflex is a response to immersion that overrides normal homeostatic reflexes. It helps conserve oxygen by directing blood flow to the heart and brain, allowing longer stays underwater. This reflex is strong in aquatic mammals like seals, otters, dolphins, and muskrats, but also exists in other mammals, including humans. Diving birds like penguins have a similar reflex. It is triggered by chilling the face and holding the breath. The cardiovascular system then constricts peripheral blood vessels, slows the pulse, redirects blood to vital organs, releases stored red blood cells from the spleen, and can cause heart rhythm irregularities in humans. Aquatic mammals have evolved further physiological adaptations.