Second sight
If our world had always been blanketed in fog, we would probably make little use of sight, and rely on our ears instead. If the fog proved opaque to radio waves, our telecommunication systems would no doubt have taken an acoustic turn, too. The ocean is a world of just this kind: visibility is poor due to the many suspended particles in the water, radio waves quickly die. And the water is full of sound: fish, marine mammals, human divers, submarines, and underwater robots all use it for subtle and complex forms of sensation, communication, and sometimes attack.
All this would be news to a scientist living only a century ago. It’s true that Leonardo da Vinci pointed out around 1490 that ‘If you cause your ship to stop, and place the head of a long tube in the water, and place the other extremity to your ear, you will hear ships at a great distance from you’, but no one took much notice of him.
It was not until World War I that listening to the underwater sounds of vessels was attempted, and not until World War II that the richness of the underwater soundscape became clear. This happened partly through two bizarre events. In 1942, acoustic buoys in Chesapeake Bay, which had been deployed to detect German submarines, all alerted at once. A ffotilla of destroyers enthusiastically depth-charged the area—but no tell-tale oil slicks appeared and the only victims were thousands of fish. Later that same year, all along the western coasts of the US, every acoustic mine (set to protect ports from anything with a propeller, in fear of a Japanese invasion) detonated at once—and again, the sole result was a multitude of dead marine life.
If the US military had known a little more about a fish called the croaker, much embarrassment and many dead fish could have been avoided. For all its undistinguished appearance, this small brownish creature has a very loud voice indeed—something like a magnified woodpecker—and croaker colonies give voice every dawn, all together, much like birds.