It all started in the 1950s when audiotaping became popular. People would record conversations, music, and nature sounds, and sometimes when they played it back, they would find tiny voices on tape - - voices that by all rational thought should not have been there . . . voices like these.
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They called it EVP, or electronic voice phenomenon. By the 1990s, tens of thousands of EVP researcher were capturing tiny voices using simply cassette tape recorders.