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Easier to detect the ferrous metal, given the size/strength requirements mean steel.

Magnetic anomaly detectors have been used since the WWII airship days. https://en.m.wikipedia.org/wiki/Magnetic_anomaly_detector

Related fact: the first Goodyear blimp was a repurposed ex-Navy submarine hunter, that likely had a MAD device fitted. You can see the restored gondola from it in the New England Air Museum.

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> given the size/strength requirements mean steel

The Soviet Alpha, Mike and Sierra classes were built from titanium. Incredibly expensive but very deep-diving and invisible to MAD.

MAD has fallen out of favor. Detection range is very short so it really only works when the aircraft is low and the submarine is shallow. There are techniques to greatly reduce a submarine's magnetic signature. The P-8A doesn't even have a MAD built in.

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