_Until recently, researchers and environmentalists who thought at all about aquatic noise were focused on marine mammals and especially whales, which can be debilitated by sonar and engine noise. But the world's 21,000 fish species also rely on sound. Many use it to communicate, and almost all rely on acoustics to navigate a dark, often turbid world. What Jacques Cousteau called "The Silent World" is actually full of natural noise, from fish calls to the sound of their bodies moving through water. To that natural din, human activity has added roughly 10 decibels of ambient commotion in the last half-century. At construction sites for offshore oil platforms, wind farms and river bridges, where explosions and pile drivers can hit 250 decibels repeatedly for months or years on end, the noise is even more intense.