From "Just Hold Your Breath" to "Don't Hold Your Breath!"
The ancients noticed that a "bell" could trap breathable air under water for brief periods, but it would be millennia before devices allowed freedom to explore. Leonardo da Vinci speculated on a system that might aid breathing and control buoyancy, and practical progress was made in the 19th century.
The first devices that allowed people to work under water used a topside supply of air, and as experience accumulated in pressurized chambers and water, symptoms of what would be later called "the bends" were recorded. By 1910, the first dive tables were devised, helping people to understand how long they could safely remain at different depths (and thus pressures). Over the next few decades, self-contained units for breathing were developed, and the introduction of the wetsuit in 1956 made recreational diving accessible to a wide range of people.
The YMCA introduced first diver-certification program in 1959, and organizations have evolved over the years dedicated to education, medical assistance, and conservation of the aquatic environment. The focus of diving has expanded from work and military uses to recreational uses. It has also shifted to some extent from food gathering to "eco-tourism;" many dive sites are preservation areas or areas where gathering or hunting requires a license and the catch is limited.
When diving equipment meant a mask and some wool long johns, diving instructions boiled down to "Just hold your breath." SCUBA (self-contained underwater breathing apparatus) allows people to breathe regularly deep below water, and this changed diving's prime directive to "Don't hold your breath!" Diving isn't just a passport to a strange and beautiful world: it's a crash course in chemistry and physics, and a basic understanding of what happens to tissues and molecules (like lungs and oxygen and nitrogen) under pressure (like under water) and in solution (that is, your blood) is crucial to safe diving.
Divers Are Certifiable
Diving is equipment intensive, and like many outdoor sports, it is inherently dangerous. Plus, it takes place in an environment in which humans cannot breathe unassisted. The equipment, of course, is what makes it possible, and dive shops require anyone who rents equipment from them to provide proof of competence to use it. This takes the form of a certification card, or "C card." (Even a modest dive rig can run into the thousands of dollars, and some components are just too heavy to travel with sensibly, so most divers rent at least some equipment much of the time.)
