When the Allied convoy escorts started to be active in 1944 in the English Channel to clear the way for the D-Day invasion, they wasted a lot of depth charges on sunken wrecks that gave sonar signals like U-boats. The Admiralty came out with a map of known wrecks to reduce the time being wasted on them. It would seem a subamrine sitting on the bottom would be picked up just as easily as the wrecks.
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