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Bay area waters were (are?) used as a naval magnetic silencing range. Most navy ships have degaussing coils that calibrate with magnetosphere

https://www.dropbox.com/s/qansmciigewti4n/IMG_2957.JPG?dl=0

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Degaussing in a bay is much different from trying to cancel out 20T(!!!) of EM signature

You don’t need to cancel it. Most electromagnetic surveillance systems have detection thresholds (e.g. in radar you have distance gate and return signal filters). Same applies for magnetically activated mines. Reducing the signature below detection thresholds is often enough. Alternatively masking the signature with another signal to confuse the detector also often works well.

EDIT: on second thought, if you can build a device to generate 20T you can probably build a contraption to cancel it out as well. It would most likely boil down to the economics or the scale of the contraption required.

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