Imagine a giant boombox dangled underwater, thumping out a kind of oceanic rap music louder than a jackhammer and able to be heard hundreds of miles away. That's a rough description of what the US Navy would like to do to maintain global superiority in the world of submarine stealth.

Even in the post-cold-war era, say Navy officials, the threat from potentially hostile subs of advanced design - able to run more silently and deeper than ever before, carrying more lethal weaponry - is growing. To counter this threat, they say, the Navy needs a new active sonar system that can locate underwater adversaries.

But according to many ocean scientists, the Navy's plans are a threat to whales, dolphins, and other marine wildlife whose own advanced sonar-based means of communication and navigation is crucial to their survival.

The controversy highlights two important trends: the Navy's growing frustration over environmental and other regulations that officials say are hampering their efforts to protect national security. And the growing problem of underwater noise pollution - caused by everything from warships to cruise liners to undersea mineral exploration - that may account for the troubling decline in some marine mammal species.

In several cases, groups of whales and dolphins have run ashore and died in the vicinity of naval maneuvers involving sonar. The Navy says it's coincidence. But others are convinced that the animals - some of which are endangered species - experienced physiological damage that disrupted their ability to communicate and navigate safely.

The National Marine Fisheries Service (responsible for protecting endangered species at sea) is considering the Navy's request to deploy the new sonar system, and the agency is expected to issue its ruling soon. Meanwhile, environmentalists and opponents in Congress and in some coastal states (including California and Maine) are fighting the request.

The Navy's Surveillance Towed Array Sensor System (SURTASS) uses underwater microphones to listen for submarines. These "hydrophones" play a passive role in sub detection.

The new and controversial addition to SURTASS involves the ability to broadcast low-frequency sounds that could help the Navy hear subs over longer ranges by "pinging" them with 18 Volkswagen-sized speakers, then listening for the echo.

According to the Navy, this Low Frequency Active (LFA) sonar enhancement to SURTASS would result in only the "remote possibility" of harm to marine mammals, sea turtles, and fish. "The numbers of animals potentially [harmed] would be so small as to have a negligible impact on the affected species' stocks and upon the availability of the species for subsistence needs," states a Navy fact sheet on the issue.

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