Many years ago, the U.S. Navy came up with a way to spy on ships and submarines from other countries. They put microphones on the bottom of the Atlantic and Pacific oceans. The microphones pick up noises in the water.
Radio waves carry the noises to land.
There, experts listen to the sounds. They try to figure out what kinds of ships are in the water. But the Navy noticed that not all of the noises were made by ships or submarines. When whales are underwater, they make noises to talk to each other. Each kind of whales make a different noise.