Q. What makes the crackling noise in the ocean that divers hear?

A. The supposedly silent world of the sea is anything but. All sorts of things make noise. Whales, dolphins, fish (they didn't name them grunt fish and croaker fish for no reason), wind at the surface, rain, earthquakes, ships, even supersonic planes breaking the sound barrier create sound waves below the waves.

But that crackling noise may be the most interesting of all.

Adam Frankel of Cornell's Bioacoustics Lab says it's made by shrimp. Snapping shrimp, to be precise.

Tiny things, much smaller than the ones we eat, they have a single claw that moves so fast in the water as it snaps shut that it does the same thing ship propellers do. It causes cavitation, the churning of dissolved gasses in the water into tiny bubbles that then collapse under the water's pressure. The crackling noise comes from the sound waves given off by those collapsing bubbles.

In areas with coral reefs, you can also hear snapping noises created when parrot fish munch away at stony coral, snapping off bits and pieces as they feed.

We can hear these tiny sounds so clearly because water is a great medium for maximizing the energy in sound waves. The speed of sound is five times greater in water than in less-dense air. So for a given amount of energy, you get much more sound.

That is why something as tiny as snapping shrimp can make warm water areas of the ocean sound like a bowl of Rice Krispies.