The world beneath the waves is a surprisingly loud place. Wind and waves, earthquakes, volcanic eruptions, and the rumbling, grinding and crashing of ice all combine to create something of an underwater cacophony. Numerous species use sound to communicate and navigate across distances from centimeters to hundreds of kilometers.
But over the last several decades, those natural sounds have in many places become overwhelmed by noises from human activities, some of them loud enough to be heard halfway across the ocean. By lowering a hydrophone into the water, says Dr. Christopher Clark of Cornell University, "I can hear seismic activity off the north coast of Brazil; I can hear it 2,000 miles away in the middle of the Atlantic Ocean."
The seismic activity to which he is referring is the search for oil and gas deposits in the seabed: seismic vessels tow an array of air guns, which release a volume of air under high pressure, creating a sound wave from the expansion and contraction of the released air bubble. I've been in a ship in the vicinity of a seismic vessel, and it was a discomfiting experience. Each blast from the air guns resonated off our ship's hull as if Thor were clanging his hammer against the side.