Divers Down Flag: Red Flag with White Diagonal Stripe

In the 1950s, and due to the growing popularity of scuba diving, James Dockery, a U.S. Navy diver, decided to design his diving flag.
 
He was inspired by the nautical flag code, specifically the “Bravo” flag.
Bravo flag alone means “loading and unloading dangerous goods” (usually explosives). So, Dockery thought that a flag similar to the bravo flag would deter ships from approaching the divers.
 
He placed a diagonal white stripe on a red square, and its use was spread by the diving magazine Skin Diver, especially among the American public. As the most widespread certifying agencies are American, more and more divers knew this flag.