A Liquid Breathing Rig involved using highly oxygenated liquid to breathe easier in areas of ocean where gasses would cause narcosis. It wasn't very popular.
In the 1940s, during World War II, French naval officer Jacques Cousteau invented the Aqua-Lung. This was the first scuba gear. Jacques Cousteau continued to improve the throughout World War II. This led to the first open circuit scuba diving technology. The word scuba was short for "Self-Contained Underwater Breathing Apparatus". It was coined in 1952 by Major Christian Lambertsen who invented the rebreather. However, rebreathers did not become common in civilian use until after the Cold War ended. The tank was usually full of air like what we have at the surface. But there was a problem. Humans could metabolize oxygen, but not nitrogen. At a certain depth, narcosis could. And if a person rose from the water too quickly, he or she would get what was known as the bends or decompression sickness. One solution to this problem was a liquid breathing rig.
Tech Level: 11
Liquid breathing experimented on in the 1960s. Animal testing was a success. Liquid breathing was prominently featured in the sci-fi movie The Abyss. In one scene of the movie, a rat is shown submerged in perflourocarbon liquid and breathing normally. This was no special effect. It was real. Perflourocarbons could hold 25 times more oxygen than human blood. Although PFCs were heavier and more viscous than gasses, the lungs could still work normally. Liquid breathing rigs could go down to 1,000 meters or so, but there was a problem. Not only did a person feel like he was drowning all the time, but an artificial respiration went down the trachea because PFCs were not good at handling carbon dioxide. This caused people to gag at first. Though cheaper than an atmospheric diving suit, it was still more expensive than normal diving equipment. Artificial gills were cheaper.