Hues from pink to orange, auburn and green draw the eye across a coral reef that has grown inch by inch for thousands of years.
     
     The underwater garden off the small island of Culebra is among the healthiest in the Caribbean Sea. But new, little-understood diseases are killing the coral, eating away its color and leaving skeletal patches that look like concrete.
     
     Across the globe, polluted runoff and algae are smothering corals while hot spells are bleaching the color from their branches and ridges.
     
     Scientists say if nothing changes, 40 percent of the world's reefs could be lost by 2010.
     
     ``I snorkel like some people go hiking,'' said Mary Ann Lucking, who runs the conservation group Coralations on Culebra. ``But it's frustrating now. Everything is dying.''
     
     Most scientists blame modern society: rampant coastal construction; polluted runoff; land clearing without erosion control; sewage; bilge water from cruise ships; removal of live coral and exotic species for aquariums; overfishing and fishing with cyanide and explosives. And that's before considering climate change.
     
     There are very few reefs, probably less than 5 percent, that bear no trace of human activity, said Mark Spalding, a marine ecologist at the U.N. Environment Program's World Conservation Monitoring Center in Cambridge, England.
     
     Spalding and other scientists have determined that coral reefs cover 110,900 square miles, less than one-tenth of 1 percent of the world's oceans. But they support more than 1 million species of marine life, sustain tourism industries and provide food for islanders throughout the tropics.
     
     Healthy reefs are like undersea rain forests that naturally draw in carbon, helping pull harmful greenhouse gases from the air. They also provide medication. AZT, a drug for HIV (news - web sites) patients, is derived from a Caribbean reef sponge.
     
     Corals come in about 1,500 known species - from soft swaying fans to stony varieties with hard skeletons that form reef bases. They are made up of polyps, tiny animals that live in colonies and feed at night on microscopic plants and creatures.
     
     The coral's surface is the living part, with color infused by single-celled algae called zooxanthellae that live in polyp tissue. The algae act like solar panels, passing energy to the coral as they photosynthesize while feeding on the corals' waste.
     
     Extremely sensitive, corals survive in a narrow range of temperature, sunlight and salinity.
     
     An uncommonly severe El Nino in 1998 raised ocean temperatures and changed currents, causing bleaching that devastated reefs worldwide. Scientists say parts of the Indian Ocean lost up to 90 percent of corals. The bleaching struck reefs around the Persian Gulf, East Africa, Southeast Asia and the Caribbean.
     
     Some recovered. Many died.
     
     Reefs least exposed to people fare best. Australia's Great Barrier Reef - so immense it can be seen from outer space - remains in relatively good condition, as do many in the South Pacific and Hawaii.
     
     Reefs popular with divers and fishermen often lose their luster. Corals die as boats smash anchors onto them, tourists touch them and fishermen walk over shallow sections.
     
     Some of the largest reef fish are shot with spear guns, and lobsters are trapped for sale to restaurants. That leaves the system unbalanced, increasing populations of smaller fish and snails that gnaw at the coral.
     
     Off Indonesia, the Philippines and other parts of Southeast Asia, some of the most severely threatened reefs face destruction from explosives hurled toward them to kill nearby fish.
     
     Scientists disagree about how much of the world's reefs have died: Some say 10 percent are degraded beyond recovery, others say 27 percent is gone.
     
     A 1998 report by the Washington-based World Resources Institute estimates people are endangering 58 percent of reefs.
     
     They also are threatened by diseases that have decimated Caribbean and Florida Keys reefs since the 1970s. It's not clear if the harmful microorganisms are new or if the coral has become more susceptible.
     
     In the Caribbean during the 1980s, white band disease wiped out much of the elkhorn - majestic corals with cypress-like branches that grow to 12 feet or more.
     
     Corals that have proved incredibly resilient, recovering after hurricanes for millennia, now are being lost at a devastating rate. Along a stunning reef on Culebra, some 35 to 40 percent of the coral has died since 1997, said marine biologist Edwin Hernandez of the University of Puerto Rico.
     
     Hernandez dived down to an area of white overtaking a golden boulder coral: ``White plague,'' he said after surfacing.
     
     Of a black band advancing across yellow brain coral, he diagnosed: ``Black band disease.''
     
     Motioning to a purple blotch on a star coral: ``That one, we don't know what it is.''
     
     Scientists have named at least 29 coral diseases.
     
     A fungus called Aspergillus - believed to have come from dust blown across the Atlantic from Africa - is eating away at Caribbean fan corals.
     
     A particularly deadly form of white plague, ``type two,'' races across corals at nearly one inch a day. Laurie Richardson, a microbiologist at Miami's Florida International University, helped isolate the cause - a bacterium of unknown origin.
     
     ``Too much money is being spent on monitoring,'' she said. ``Understanding what causes these diseases is the first step in understanding how to manage or prevent them.''
     
     The critical priorities are to reduce harmful fishing and pollution, said Clive Wilkinson of the Global Coral Reef Monitoring Network, based in Australia.
     
     ``If the pressures are taken off reefs, they will recover naturally and relatively rapidly,'' in 10 to 20 years, he said.
     
     Some conservation efforts are paying off. After years of insistence from local fishermen, Puerto Rico set up a ``no-take'' area off Culebra. Two years later, fishermen say their catch has improved, and healthier fish populations mean healthier reefs.
     
     Even as white plague eats away at boulder corals off Culebra, they still spawn once a year. On the seventh night after a recent full moon, one released peach-colored eggs that floated away to form a new colony. Whether it will thrive is an open question.
     
     Perhaps the most difficult threat to assess is climate change: Warmer ocean temperatures would bleach reefs and it's uncertain how weakened corals would cope with higher sea levels.
     
     The costs involved in protecting the world's reefs also remain unclear, but many experts warn the costs of doing nothing are much greater. By one estimate, coral reefs are worth $375 billion each year to fishing, tourism and other industries.
     
     Along the north coast of Puerto Rico, where sewage and sediment have killed most reefs, some outcroppings of elkhorn and brain coral survive off San Juan.
     
     To save these and other reefs, experts say, people will have to change.
     
     ``Either people realize they have to press politicians to fight for these things, or we lose,'' Lucking said. ``We lose everything.''
     
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     On the Net:
     
     Coralations: http://www.coralations.org
     
     UNEP: http://www.unep-wcmc.org/marine/coraldis
     
     Coral Reef Monitoring Network: http://www.gcrmn.org
     
     Coral Reef Initiative: http://www.icriforum.org
     
     World Resources Institute: http://www.wri.org/indictrs/reefrisk.htm
     
     ReefBase coral information: http://www.reefbase.org/default.asp