Short answers, pulled from the story.
The Mark 24 mine was actually an air-dropped acoustic homing torpedo, not a mine. The United States Navy called it a mine deliberately to conceal its true capabilities as an anti-submarine weapon.
FIDO sank 37 Axis submarines and damaged 18 during the war, achieving an effectiveness rate of approximately 18 percent. This compared favorably to the 9.5 percent effectiveness rate of aircraft-launched depth charges.
FIDO used four piezoelectric hydrophones operating at 24 kHz to passively detect the sound of a submarine's propellers. Once the signal exceeded a preset threshold, a proportional homing system steered the torpedo toward the source.
Development was shared among four teams: Bell Telephone Laboratories, the Harvard Underwater Sound Lab, General Electric, and Columbia University's Special Studies Group. The project was designated Project 61 under the Office of Scientific Research and Development.
The Mark 24 entered Allied service in March 1943. Approximately 4,000 were produced, reduced from an initial order of 10,000 because the weapon's performance exceeded expectations.
The Mark 27, nicknamed Cutie, was a variant developed for submarine use against surface vessels, inverting FIDO's role. It entered service in the Pacific in the summer of 1944, and its first combat success came when the submarine Sea Owl damaged a Japanese patrol vessel in the Yellow Sea in November 1944.