The Mark 24 mine (also known as FIDO or Fido) is an air-dropped anti-submarine warfare weapon (ASW) incorporating passive acoustic homing system and torpedo integration. It was used by the United States, the British and Canadian forces during the Second World War and entered service in March 1943 and remained in use with the US Navy until 1948. Approximately 4,000 torpedoes were produced, with 340 ultimately being deployed during the war. Two-hundred and four torpedoes were launched against submarine targets, with 37 Axis submarines being sunk and a further 18 damaged. The deceptive name of "Mark 24 mine" was deliberately chosen for security purposes, to conceal the true nature of the weapon.