You’ve probably seen the movie. Submarine crewmen look up anxiously. Suddenly there’s a terrific explosion. Lights flicker off and on. Streams of water spurt from leaks. The attackers work the submarine over again and again, straining the crew to—or nearly to—the breaking point. Sometimes the submarine crawls away; sometimes the next scene show her survivors on board the attacking ships. In either case, you’re reminded that antisubmarine warfare, particularly during World War II, usually meant depth charging. However, you’ve probably never seen a live depth charge because this weapon died off with the advent of fast submarines in the postwar years.
During World War I, when the depth charge was invented, a submerged submarine was an elusive target, usually only indicated by the swirling water where a periscope had been or by the track of a torpedo. The only prewar antisubmarine weapon was a sweep, a towed submerged explosive developed by the Royal Navy. If a destroyer towing a sweep happened to pass over a submarine and the device snagged the boat, the destroyer could detonate the explosive electrically. But that rarely happened.