The Magnetic anomaly detector, or MAD for short. Scan for differences in the planets magnetic field which are caused by the metallic makeup of submarines. In from the depths, this would allow low flying aircraft to detect submarines in a similar way to that of wireless snoopers. The more metal or HA blocks in a sub craft, the stronger the signal. Note that this is most effective against shallow diving subs and deep submarines can easily evade the detection.
The advantage of this is a detection system that can be kept out of the water, sonar buoys can be jammed or intercepted by enemy defenses. Meaning this can be a more reliable source of detection. On the contrary, this system struggles against small or deep submarines that do not have a significant makeup of metallic blocks, meaning the system can still be countered in ways where sonar buoys would be the better option.
Wikipedia Article on the subject
https://en.wikipedia.org/wiki/Magnetic_anomaly_detector
A magnetic anomaly detector (MAD) is an instrument used to detect minute variations in the Earth's magnetic field. The term typically refers to magnetometers used by military forces to detect submarines (a mass of ferromagnetic material creates a detectable disturbance in the magnetic field). Military MAD equipment is a descendant of geomagnetic...
