Listening with a hydrophone is an incredible experience. You dip it down into the water and suddenly you’re connected to this massive hidden world. You become liquid. You hear the tiniest sounds like air bubbles and shrimp and also powerful sounds like cargo ships miles away.
I like environments where it is difficult to hear. Difficult, not because it is loud, but because we are not made for a particular listening environment. When a human ear is underwater, the ear drum does not vibrate the same way that it does in air. The ear barely works under water, so the way we hear is actually through bone conduction. Pressure in water is translated to our skull, which transmits the sound directly to our inner ear.
In water, sound travels four times faster than it does in air. It’s a thick medium for sound, which means sound travels well and quite far. It also means certain sounds are not able to move far, such as high frequency sounds. Above a few thousand kilohertz, sound dies off shortly after it is emitted.
In 2011 I worked on a scheme to set up hydrophones along the perimeter of the Bay. Hydrophones can be built using a simple DIY recipe. See this page from Leafcutter John.