Powerful underwater sonar creates tissue-destroying gas bubbles in the vital organs of whales and other marine mammals, causing a fatal sickness similar to the "bends" that deep-sea divers undergo when they surface too quickly, a new study contends.

For the first time, scientists say they have pinpointed the reason that whales beach themselves and die after exposure to certain types of sonar.

The study was based on an international naval exercise in the Atlantic a year ago that caused the stranding of 14 beaked whales on beaches in the Canary Islands. Analysis of the whales within hours of their exposure revealed the cause, according to a team of British and Spanish researchers.

The group's solution to the long-standing mystery has just been published in the scientific journal Nature.

In another incident more than three years ago, a group of 17 medium-sized beaked whales stranded themselves in the Bahamas when U.S. Navy ships were conducting sonar exercises nearby. Seven of the mammals died within hours.

That episode led to a major investigation by scientists from the Navy and the government's National Marine Fisheries Service. Although their report directly blamed the sonar signals for mass strandings, it concluded that more research was needed to explain why the sonar had disrupted the whales so severely.

Daniel Costa, a marine-mammal expert at the University of California-Santa Cruz, who has consulted with a project using low-frequency sonar signals to study deep-water temperature changes, said Wednesday that he and most of his colleagues agree that the far more powerful and relatively high-frequency sonar waves used by naval vessels are in fact the cause of mass whale strandings.

However, he said he considers that the evidence proposed by the British and Spanish researchers provides only a "highly tenuous" explanation for the disorientation and death of the deep-diving whales.

Other California experts, uncertain about the proposed explanation, referred a reporter's inquiry to Robert Gisiner of the Office of Naval Research in Arlington, Va., who is a leading expert on what has long provoked a major controversy among environmental organizations. A spokesperson for the agency, however, would not allow Gisiner to speak to reporters.

The team of British and Spanish scientists proposing the explanation for last year's mass stranding was led by Paul Jepson of the Institute of Zoology in London.

He and his colleagues said there was no evidence of bacterial disease in the whales. Autopsies showed that their blood vessels, livers, hearts, kidneys and fatty tissues were filled with large and rapidly expanding gas-filled cavities caused by the sonar pulses, the scientists said.