Episode Details

In 1878 engineer Henry Fleuss built the first commercially practical closed-circuit rebreather: a rubber mask, a breathing bag, pure oxygen from a copper cylinder, and rope yarn soaked in caustic potash to scrub carbon dioxide. It gave up to three hours underwater with no bubbles, so British, Italian, and German frogmen adopted the technology in World War II. In the US, Major Christian J. Lambertsen built a free-swimming oxygen rebreather in 1939 and in 1952 patented an improved version under the name SCUBA. Pure oxygen, though, becomes a neurotoxin under pressure and causes sudden convulsions. In 1942 Jacques Cousteau and Emile Gagnan created the open-circuit Aqua-Lung with a demand valve, and in 1952 Ted Eldred's single-hose design set the pattern divers still use.

Water rewrites the senses. A mask's air space lets the cornea focus but makes objects look about 34 percent bigger and 25 percent closer, and red light vanishes within 15 to 20 feet, so blood looks green at 100 feet. Boyle's law makes holding your breath on ascent deadly, since expanding air can rupture the lungs and cause an arterial gas embolism. Neutral buoyancy is a positive feedback loop as a wetsuit and BCD compress with depth, so skilled divers fine-tune it with their lungs. Henry's law drives nitrogen into tissue, causing the bends on fast ascents, and deep divers answer with blended gases.

• Scuba is an acronym for self-contained underwater breathing apparatus.
• Nitrox at 32 or 36 percent oxygen extends bottom time but shrinks maximum depth.
• Trimix and heliox replace nitrogen with helium to avoid narcosis on deep technical dives.
• Technical divers may carry four to six cylinders and switch gases as depth changes.
• Modern closed-circuit rebreathers use computers and oxygen sensors to inject gas in real time.