The underwater acoustic environment is quite different from the one most people experience as part of their daily lives. The physics of underwater sound creates effects that the uninitiated might find unusual.
Whales use the sound channel to communicate over vast distances.
Among man's first uses of this phenomenom was the location of downed aircrew. Operators at these ranges could use the differing time of arrival of these sounds to calculate an approximate position of the aircrew. Click on the link below to hear an example of this sound. Underwater charge from thousands of miles. This is due to dispersion.