MADs (Magnetic Anomaly Detector) are frequently used in ASW warfare to detect submerged submarines. These detectors are mounted to helocopters or other low altitude aircraft which are then flown in a search pattern over the ocean. Until recently a submarine found with a MAD was pretty much screwed since ASW aircraft also carry torpedos to drop on submerged targets. But the new Seawolf class boats can launch SAMs from periscope depth and shoot back at the offending aircraft.

MADs are sometimes called MAD detectors, don't ask me why; I think it's redundant too.