There's a layer in the ocean - in between the warm surface waters and the deep, high pressured waters - where sound waves move more slowly. The so-called SOFAR channel is a sweet spot of ocean acoustics.
American and Soviet researchers independently discovered the channel in the 1940s. The U.S. military deployed hydrophones in the underwater channel for surveillance purposes, and today still uses them for scientific research.
From the Here & Now Contributors Network, Todd Bookman of WHYY's The Pulse reports.
This segment aired on February 12, 2015.