Residents of Hanalei Bay on the Hawaiian island of Kauai woke up last weekend to a distressing sight: As many as 200 melon-headed whales, a small and sociable species that usually stays in deep waters, were swimming in a tight circle as close as 100 feet from the beach, showing clear signs of stress.

To keep the animals from beaching, the locals kept a vigil all day and through the night, until a flotilla of kayaks and outrigger canoes could be assembled to herd the animals back out to sea. So far, only one young whale has been found dead.

But among increasingly worried whale advocates and researchers, the event set off immediate alarm bells: Melon-headed whales are not known to beach themselves, and nothing like this mass stranding close call has occurred in Hawaii for 150 years.

Attention quickly focused on the Navy and its use of active sonar -- a wall of sound sent out to find underwater objects that can reach the decibel levels of a jet engine. Sonar has been implicated in several recent mass whale strandings around the world, and the latest research has strengthened the association and suggested that the number of incidents may be far greater than anyone realized. The most recent study found that over the past 40 years, mass strandings of the most noise-sensitive whales off Japan occurred repeatedly in the waters near a U.S.-run naval base, but were unknown in comparable areas elsewhere.

Several hours after the Hanalei Bay episode began, locals learned that a six-ship Navy fleet 20 miles out to sea had begun a sonar exercise the morning that the melon-headed whales headed toward shore.

Officials at the National Oceanic and Atmospheric Administration said it is too early to conclusively link sonar to the near-stranding, but they said their top priority is to learn more about the Navy exercise.

Unless a different and convincing explanation can be found, the Hawaii incident is destined to become the newest case in a high-stakes battle between environmentalists and the military over a technology that has been a staple of Navy operations for decades. Marine mammal advocates say it has become increasingly apparent that sonar can lead to death for whales, porpoises and other sea creatures, and something must be done to limit its toll.