Correct. The magnetic signatures seen are the <i>permanent magnetic</i> moments embedded in the steel of the ships, which are picked up by passive magnetic-anomaly detectors. These have to be removed by randomization in a normal all-at-once degaussing process.<p>It is hypothetically possible to have an active-detection process too, but presumably that would take the form of emitting a pulse of electromagnetic radiation and looking for its echoes, in other words it&#x27;s just another form of vision. The standard problem applies: find a frequency at which water is mostly transparent. Unfortunately the absorption spectrum for water is <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Electromagnetic_absorption_by_water#&#x2F;media&#x2F;File:Absorption_spectrum_of_liquid_water.png" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Electromagnetic_absorption_by_...</a> , it has a minimum in the visible range.<p>In addition to being one of the more stunning confirmations of evolution (of <i>course</i> our eyes are only sensitive to the least-absorbed wavelengths of light in water -- our ancestors evolved eyes in the ocean! there weren&#x27;t many other useful wavelengths down there!) this has the nasty property that it&#x27;s unlikely that we&#x27;d be able to detect a sub this way with any better approach than shining a bright laser or spotlight or what have you, and watching what&#x27;s reflected back with our own eyes.