During WWI, German submarines (U-boats) were a real menace, sinking thousands of Allied merchant ships and seriously damaging supply lines. In 1917, two Canadian physicists by the names of Robert William Boyle and Albert B. Wood who were working under the British Board of Invention and Research, developed an apparatus for underwater echo ranging using ultrasound, which was known as ASDIC, and was the world’s first practical underwater active sound detection apparatus. It was developed for the British Navy’s Anti-Submarine Division, under a shroud of secrecy.
This technology evolved into sonar which proved vital in WW2, and the same technology would also eventually by used in the development of medical ultrasound imaging.