The Navy is moving ahead with plans to build a 500-square-mile sonar training range off the coast of North Carolina, officials said last week. The project has sparked fierce opposition from environmentalists, who say some of the world's most endangered whales and sea turtles pass through the area.

Planning for the $99 million range has been underway for almost 10 years, but environmental challenges and concern that the sound waves from sonar may harm protected marine mammals have held up the process. The Navy published its draft environmental impact statement Friday and plans to begin a series of public hearings on the proposal next month.

The proposed site, about 50 miles off North Carolina, was selected to provide the Atlantic fleet with training in the use of sonar in coastal areas, where the Navy believes the greatest submarine threats now exist. The global spread of quiet and relatively low-cost diesel submarines has alarmed the Navy and convinced officials that its sailors need more training in detecting hostile subs in canyons and ocean beds closer to shore.

But animal researchers and environmentalists have grown increasingly alarmed over the Navy's plans and the potentially damaging effects of active sonar -- which sends out very loud blasts of underwater sound.

Whales and other marine mammals have very sensitive hearing, and a growing body of research has shown that sonar can disorient and sometimes kill them. The Natural Resources Defense Council, an environmentalist group, sued the Navy last week over its use of mid-frequency sonar, the type that would be deployed at the new sonar range. The group claimed that the sonar threatened endangered animals, in violation of several federal environmental laws.

Advocates for marine mammals say they see the proposal for an East Coast sonar range as a long-feared "test case" of increased Navy assertiveness -- especially because one of the most endangered and highly protected whales on Earth migrates through the region.

The world's 300 to 350 remaining North Atlantic right whales, whose numbers were decimated in the 1800s by whalers who considered it the "right" one to harpoon, are known to travel from the Arctic to Florida along the East Coast. Their plight led this year to federal regulations requiring Navy and commercial vessels to take a variety of steps to avoid them.

"These animals are teetering on the brink of extinction," said Sharon Young of the Humane Society of the United States, on Cape Cod. "Adding a sonar range in what may well be the middle of their migration route is just insane."

In its draft environmental impact statement, the Navy says that right whales pass through two of the possible sites for the training range -- off Virginia and another off Florida. But for its preferred North Carolina site, located between the other two, the document says only that humpback and sperm whales could be harmed, saying nothing about right whales.

Asked why the right whale was not mentioned as a potential problem at the North Carolina site, a U.S. Fleet Forces Command spokesman said the majority of right whale sightings there are within 37 miles of shore. Because the training range would be 50 miles offshore, the environmental impact statement concludes the right whale is "expected to occur only rarely in the vicinity of the proposed site."

The exact migration patterns of the North Atlantic right whale are not well documented, but many experts believe they must pass along the North Carolina coast on their way from Canada to their calving ground off northern Florida. "There is no evidence at all to say that the right whale migration is closer than 50 miles to shore through the Carolinas," said Doug Nowacek, a marine mammal specialist at Florida State University. "We just don't know where they go."

Young said the Navy's site assessment is "either hopelessly naive or disingenuous."

While the right whale is currently the focus of the controversy, other marine mammals also could be affected by the range. The Navy said in its application for the sonar range that it will ask the National Marine Fisheries Service for authorization to disturb or "harass" spotted, bottlenose, common, Risso's, and Clymene dolphins, and pilot, humpback and sperm whales.

In its draft statement, the Navy presented both its reasons for building the range and its assessment of the environmental risk. The primary need, the statement said, is to train sailors in the proper use of sonar.

The Navy said that after an exhaustive review, it concluded that "the overwhelming majority" of noise would be in the "non-injurious" range. Overall, it concluded, sonar noise would have a "negligible impact" on marine mammals and new procedures could be put into practice to further limit any potential risks.

Adding to the controversy, the proposed North Carolina site is in the general area of a mass whale stranding that occurred in January, when 37 whales from three species died on the beach within 24 hours. The Navy was conducting a sonar training exercise offshore during that time, but Navy officials say the ships were too far away to have caused the strandings.

A National Oceanic and Atmospheric Administration investigation of the stranding was to have been issued this summer, but officials now say it will not be ready until early next year, after the comment period for the sonar training range has closed.

Joel Reynolds, director of the NRDC marine mammal protection program, said the Navy is taking an unnecessary risk.

"The scientific evidence that active sonar kills whales is unequivocal," he said. "If the Navy wants to make North Carolina an epicenter for training with this dangerous technology, it must first show that we won't see more whales on North Carolina beaches because of its actions."

The sonar training range would consist of a web of underwater sensors, cables and submarine pathways, and exercises typically would involve surface ships, airplanes and helicopters. The plan also envisions dropping almost 8,000 floating sonobuoys, some of which send out the same kind of loud "pings" as do ship- and submarine-based sonar.

The range would be designed to use active mid-frequency sonar, which has been used for decades. Researchers first linked this kind of sonar to whale strandings in the mid 1990s, after a NATO exercise off Greece.

Since then, the connection between the sonar and marine mammals -- especially deep-diving animals like the beaked whale -- has grown stronger. After a mass whale stranding in the Bahamas in 2000, the Navy concluded that its sonar was the most likely cause of the deaths. Whale and porpoise strandings off Hawaii, the Canary Islands, Washington state and Japan also have been linked to sonar exercises, but not with the same degree of certainty.

The Navy has another training range off Hawaii, but officials said it is generally not available to ships in the Atlantic fleet and does not provide the kind of coastal, shallow-water sonar practice now considered necessary. The North Carolina site, they said, is needed because of the "clear and present threat posed by quiet diesel electric submarines to our carrier strike groups, amphibious task forces, and to the sailors and Marines stationed aboard them."