Pardon my ignorance but if there was something at the bottom of the ocean surely the MAD in the Orions would find it?
Nope:
To reduce interference from electrical equipment or metal in the fuselage of the aircraft, the MAD sensor is placed at the end of a boom or a towed aerodynamic device. Even so, the submarine must be very near the aircraft's position and close to the sea surface for detection of the change or anomaly. The size of the submarine and its hull composition determine the detection range.
From:
Magnetic anomaly detector - Wikipedia, the free encyclopedia