The underwater telephone, also known as UQC, AN/WQC-2, or Gertrude, was developed by the U.S. Navy in 1945. The UQC underwater telephone is used on all manned submersibles and many Naval surface ships in operation. Voice or an audio tone (morse code) communicated through the UQC are heterodyned to a high pitch for acoustic transmission through water.
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Underwater telephone Wikipedia(Text) CC BY-SA