Rising surface water temperatures and pollution place coral reefs at risk worldwide.

Slipping beneath the surface of the clear Caribbean, divers see a vast landscape of colorful coral boulders that spread across the sandy shallows 30 feet below.

This offshore island is far from the sewage outfalls of Belize City and the crowded dive sites of the northern barrier reef. Unlike most of the Caribbean these days, its corals are considered pristine. They spread out on the lagoon floor and over the cliff-like reef wall beyond: corals shaped like giant cauliflower heads, thickets of staghorn and finger corals, and occasional antler-like colonies of elkhorn.

Only there's something wrong. The elkhorn corals are dead and covered in algae - victims of a regionwide die-off. Other corals have bleached completely white, dotting the bottom conspicuously all the way out to the edge of the lagoon.

"It's amazing how much coral bleaching we're seeing today," says Jonathan Kelsey, a Peace Corps volunteer working at the University College of Belize's marine research station here. "A few weeks ago when we dove here, all of these corals looked completely normal. I'm concerned they won't recover."

Coral reefs are under severe threat in much of the world from a variety of stresses - many of them the result of human activity: fishing, pollution, silt from the land, and damage from anchors and careless divers. And according to marine biologists, possibly the greatest threat is global climate change causing sea temperatures to rise. Warm water has degraded large areas of these so-called rain forests of the seas. By one estimate, 10 percent of the world's reefs are dead or beyond recovery and another 30 percent may follow over the next 20 years.

This year brought the hottest surface temperatures since 1982, according to National Oceanic and Atmospheric Administration satellite data.

Southeast Asian reefs are believed to be the most threatened because of intense population pressures on their resources. In the Philippines, desperate fishermen have for decades dropped dynamite and cyanide on the reefs to stun or kill the fish within. That, combined with pollution from the land, is estimated to have damaged or killed 90 percent of the country's 34,000 square kilometers of reef. In Malaysia's portion of the island of Borneo, a 1997 reef survey found that 99 percent of the reefs had been damaged by blast fishing.

Half the coral reef systems of the United States, American Samoa, and the Virgin Islands are at risk, according to the National Oceanic and Atmospheric Administration. Tourists flock to many Caribbean island nations to dive and snorkel on the reefs, prompting seaside developments that increase erosion and pollution stress on the corals.

The declines are troubling, and not merely for aesthetic reasons. Millions of people depend on reefs for food, livelihood, and protection from storms. Many Caribbean and Pacific island nation economies depend on reef-based tourism; in Belize it's the largest foreign-exchange earner and central to future development plans."We're seeing the collapse of reef systems on a local scale like Jamaica or the East Coast of Panama," says Jeremy Jackson, a coral specialist now at the Scripps Institution of Oceanography in La Jolla, Calif. "Its hard not to be cynical when over the course of your career you've seen so many beautiful reef systems deteriorate and die all over the Caribbean."

Coral reefs are created through a cooperative alliance of coral polyps - a tiny animal that filters detritus from the surrounding water - and single-celled plants that live within the polyps. The plants generate many of the nutrients the corals need to live and capture calcium from the water, which the polyps use to build their limestone shells.

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