Sonar technology was utilised effectively during World War 2 as a means of detecting enemy submarines while they are deep under water in enemy territory. By sending out pulses of sound which would rebound off objects and return to the ship, crew members could determine the distance of an enemy vessel and prepare for a counter-attack. Unfortunately, sonar was only effective for vessels underneath the ocean at large deeps, so while submarines were close to the surface they could travel undetected.
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