Free diving is an unconventional form of unassisted descent into the depths of a body of water, usually in an ocean or lake setting. Free diving is usually classified as an extreme sport, one of an ever-increasing number of athletic activities in which competition with other participants is secondary to personal risk-taking and the sense of achieving a personal objective. Hang gliding, heli skiing, and rock climbing often include the dangers associated with extreme sports. For many free-diving enthusiasts, the purpose of the sport is to explore underwater worlds where the diver is unencumbered by scuba gear and the attendant equipment noises that drives away marine life. The International Association of Free-Divers (IAFD) is based in Grand Cayman Island, in the Caribbean, a place famed for its many reefs and diving venues.
The fundamental object in free diving is to determine how far below the surface or for how long the diver can function on a single breath of air. In competitive free diving, where the athletes seek to travel to the greatest depth on a single breath, the sport is often referred to as competitive apnea. Apnea is the medical term for the cessation of breathing.
As with any other sport, there are techniques which may be practiced to increase the diver's ability to successfully remain under water for an extended period of time. The simple method of practicing the holding of one's breath for ever-increasing periods is the first and most basic practice element in the sport. Prior to the actual execution of a free dive, many divers will perform a series of hyperventilations, where they inhale and exhale at a much faster than normal rate. Hyperventilation tends to artificially depress the amount of carbon dioxide (CO2) remaining within the body, as the cardiorespiratory system is repeatedly cleared of the gas by the rapid breathing process. It is the presence of CO2 that otherwise triggers the body's autonomic nervous system to stimulate breathing, the process regulated by the hypothalamus region of the brain; hyperventilating temporarily blocks the effect of this mechanism.
Free divers are not required to be technically proficient swimmers but for both safety and to descend into the water as far as one possibly can, the free diver must be a powerful swimmer. The increased pressure on the body imposed by deep water is significant. At sea level, the regular pressure of the air upon the skin is 14.7 pounds per square inch (psi); each 1 ft measure of water depth increases the pressure upon the body by 0.43 psi. At a depth of 100 ft (30 m), the water pressure will total 43 psi. Given the physical risks associated with increased water pressure on the body at the depths to which a free diver may descend (depths in excess of 150 ft [45 m] by free divers are common), lean muscle mass and an overall high level of general fitness is required. Free diving and the associated stresses on both the cardiorespiratory and the cardiovascular systems pose significant problems for persons with poor fitness, especially because the diver's heart rate will slow (bradycardia), and the blood vessels constrict to reduce blood flow to the limbs and retain as much blood as possible in the vicinity of the internal organs.
Free diving may be performed with a snorkel, a form of breathing tube used by swimmers under water. In most types of this sport, the diver wears a mask; in many categories the competitors wear fins to assist them with their propulsion during ascent and descent. In its competitive forms, free diving is organized into one of three categories: constant weight, variable weight, and no-limit diving. Constant weight diving requires the diver to maintain the same weight, assisted or natural weight, throughout the entire dive.
In the variable weight categories, the divers are permitted to carry up to 66 lb (30 kg) of ballast to assist with their descent; the ballast can be released when the diver reaches their underwater objective.
No-limit free diving permits the diver to descend with a weighted line and sled, with the return to the surface assisted by an air-filled device. In 1999, Umberto Pelizzari of Italy established what was accepted by the IAFD was the deepest free dive of all time by descending 150 m (495 ft), in a total elapsed time of 2 min 57 seconds. Patrick Musimu of Belgium descended to a depth of 689 ft (209 m) in the Red Sea, in 2005.
As befits its reputation as an extreme sport, free diving poses a number of risks of serious injury or death to its participants. A shallow water blackout is the expression used to describe cerebral hypoxia, a condition caused by a lack of oxygen being received into the brain. The body consumes all of its stored oxygen in the process of the energy production needed to propel the body underwater. The body's production of CO2, a natural byproduct, continues to build to a point where the body is not able to release it, due to the effect of a hyperventilation performed prior to the dive.
A deep water blackout generally will occur when the diver has swum to a depth of greater than 150 ft (45 m), resulting in a partial pressure drop during the diver's ascent to the surface. In these circumstances, the diver's lungs, which contracted in response to the greater water pressure experienced as the diver descended, maintain their reduced partial pressure on ascent, causing the blackout condition.