Most whales and many other marine species depend on sound as they hunt for food, avoid predators, find mates, and maintain their awareness in the darkness of the sea. But over the past century the acoustic landscape of the ocean has been transformed by human activity -- intensely loud military sonar, oil-and-gas surveys, and the ever-increasing traffic of commercial ships. This noise can have impacts on marine life ranging from long-term behavioral change to hearing loss to death. This November 2005 second edition of NRDC's groundbreaking 1999 report on ocean noise has been completely rewritten to reflect the rapid growth of the scientific record. It reviews the science, surveys the leading contributors to the problem, and suggests what might be done to reduce the impacts of noise on the sea -- before the proliferation of noise sources makes the problem unmanageable.