Every veteran of submarine war patrols has stories of false "enemy contacts" reported by underwater detecting devices. If the signals were only reflections of the high-frequency sound waves sent out by the sub itself, false alarms could easily be caused by whales or schools of fish. But far more baffling were the cases in which a different sound impulse was recorded. This, it seemed, might be the enemy's own detection device at work. Many a crew was called to battle stations ready for deep-sea combat, only to learn that the signals had been...
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