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.