SCUBA (or scuba) is a system of air tanks and a regulator that lets you breathe underwater. It was invented for the US military by Dr. Christian Lambersten just prior to World War II. He designed the apparatus for the United States military in 1939 that he called the "Self-Contained Underwater Oxygen Breathing Apparatus." The military shortened this to the acronym "SCUBA."
Four years later, improvements were made by Jacques Cousteau and French engineer Émile Gagnan (see demand regulator).