And they change the density. Finally, the density of water determines how sound travels through it. Those density variations do a strange thing to the spread of sound in the ocean. Sounds go out in all directions in air. But sound that originates at a particular ocean depth spreads only outward.
Sound stays in a plane, if it has a certain density pattern.
Sound also travels five times faster in water than in air. And it doesn't attenuate as fast. So they'll lower a loudspeaker near an island in the South Indian Ocean. They'll broadcast a low-frequency tone. Listening posts will pick up the sound three hours later, when it reaches all the way to America.