Listening practice: The ocean channel that carries sound across oceans

Imagine dropping a sound into the open ocean. You might expect it to spread in a straight line and slowly disappear. But the ocean has a natural sound channel that can carry low-frequency signals for hundreds, and sometimes thousands, of miles. NOAA calls it SOFAR, pronounced like the words "so far." The name comes from Sound Fixing and Ranging. In a wartime test described by NOAA, scientists placed a hydrophone, an underwater microphone, near Woods Hole, Massachusetts. A ship about nine hundred miles away released an underwater explosive, and the hydrophone detected the sound. The sea had carried a signal almost too distant to imagine.