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Scuba diving is a popular sport worldwide and involves entering a body of water and remaining below the surface.  The word "scuba" is an acronym for "Self-Contained Underwater Breathing Apparatus."  Divers carry an air tank which allows them to breathe while submerged.  Scuba diving most commonly takes place in the ocean, but divers can also participate in certain lakes, rivers, and ponds.  Snorkeling is a shallower form of dive sport, taking place on the surface or just below.

Depending on the location and type of diving, divers may be able to enter the water from shore, or they may need to dive from a boat.  One or more people remain onshore or in the boat as a spotter for the group.  They must fly a red flag with a white diagonal strip to alert nearby boats when divers are underwater.

Scuba enthusiasts must become certified through a scuba diving agency. Well-known agencies include PADI, the National Association of Underwater Instructors (NAUI), and Scuba Schools International (SSI).  Participants must be confident swimmers, in good health, and at least 12 years old.  The courses teach use of diving equipment, pressure equalization, efficient swimming underwater, and safe ascension through class work, pool practice, and open water dives.

Unless scuba divers ascend at a steady rate, they risk decompression sickness, also known as the bends.  This occurs because the body absorbs nitrogen which forms tiny bubbles in the bloodstream which remain trapped in the body if the diver ascends too quickly.  This can cause extreme pain in joints and organs, and can be fatal in severe cases.  Decompression sickness can be avoided by gradual ascension and calculation of how long to stay at certain depths.  Divers must also wait 12 or 24 hours before flying on a plane due to rapid changes in air pressure.