Early ideas of autonomous under-water systems appear in Twenty Thousand Leagues Under the Sea.
Scuba diving is the use of independent breathing equipment to stay underwater for long periods of time for recreational diving and professional diving. The diver swims underwater, but walking and the use of diver propulsion vehicles is possible while breathing from scuba equipment. The word 'SCUBA' is an acronym for "Self Contained Underwater Breathing Apparatus", but it is grammatically acceptable to refer to 'scuba equipment' or 'scuba apparatus' in conversation.
The two types of scuba equipment are the "open-circuit" Aqua-lung, developed by Emile Gagnan with assistance from Jacques-Yves Cousteau and the "closed-circuit" rebreather.
Free-diving refers to various aquatic activities that share the practice of breath-hold diving. Examples include breathhold spearfishing, freedive photography, Apnea competitions and, to a degree, snorkeling. The activity that garners the public's attention is Competitive Apnea, considered an extreme sport, where divers attempt to reach great depths on a single breath and without direct assistance of breathing apparatus.
Snorkeling is the practice of swimming at the surface of a body of water while equipped with a diving mask, a shaped tube called a snorkel, and usually swimfins. Combining these tools allows the snorkeler to observe underwater attractions for extended periods of time with relatively little effort.
Snorkeling is a popular recreational activity, particularly at tropical resort destinations and many places recreational scuba divers venture (see: snorkeling locations). Snorkeling is also used by scuba divers when near the surface, and it is used in some water-based search and rescue missions.
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