Early bell suits that contained air hoses allowed people to function underwater. These bell diving suits were heavy and dangerous. Later systems relied upon compressed air and regulators, yet these were still large and impractical.
In 1942 Nazi-occupied France, Frenchmen Cousteau and Gagnan invented the first practical underwater breathing apparatus, Aqua-Lung.
In their system, called open-circuit, the air is expelled as the diver breathes out. This is sub-optimal for commandos, because the bubbles may reveal their location. However, it functioned well enough that Cousteau used the system to fight fascist Italy.
In 1952, American Major Christian Lambertsen invented and patented a closed-circuit “rebreather” used by underwater commandos since it does not create bubbles. He named this Self Contained Underwater Breathing Apparatus, or SCUBA. Eventually, SCUBA became the term for both closed and open-circuit systems.
Cousteau made early underwater films during the war and increased his visibility after the war, becoming a well-known advocate ocean environmentalist.