In 1987 marine scientists were alarmed to discover extreme bleaching of coral reefs in the Caribbean. For a time it was thought this might be further evidence of global warming. Although the debate in this area was inconclusive, it succeeded in drawing attention to another global trend of unquestionable importance, namely the decline of coral reefs.

The reefs are already suffering from a broad array of stresses; silt from deforestation and coastal pollution from crowded coastlines are choking them; and over-exploitation by coal miners, fishers and tourists are destroying and depleting them. A study from the World Conservation Union and the UN Environment Programme in the mid-eighties found that people had damaged or destroyed significant amounts of reef off the coasts of 93 countries. Reefs, it turns out, are among the most endangered ecosystems on earth.

They are also among the most precious. Coral reefs are underwater marvels of flourescent colours, fantastic shapes and improbable creatures; delicate purple sea fans, blood-red sponges, blue-spotted groupers, spiny pufferfish, snorkel-nosed moray, poisonous scorpionfish, giant clams, yellow-lip snakes, and giant manta rays. "The eye struggles to make sense of the confusions of colour, motion and shape that assault it," relates Kenneth Brower, an environmental writer. "Nothing in the temperate zone can prepare the retina, or mind behind, for what goes on under the surface of shallow tropical seas." For their beauty alone, reefs are one of the treasures of the earth.

This showy display, however, merely hints at greater riches. Coral reefs are among the most biologically diverse ecosystems on earth, holding a substantial portion of the biological underpinnings of life on the planet. They form what are thought to be the most species-rich ecosystems in the oceans, the crucible of life some three billion years ago. Covering only 0.17% of the ocean floor, an area the size of Texas, coral reefs are home to perhaps one quarter of all marine species, earning them the title "the tropical rainforests of the oceans". Only the vast, little-explored deep ocean floor could rival their marine biological diversity.

Like rainforests, reefs hold considerable untapped potential to contribute to science, particularly medicine. The intense crush of life spawns unique chemicals such as kainic acid, collected from reef organisms in Japan and Taiwan. Kainic acid is used as a diagnostic chemical to investigate Huntingdon's Chorea, a rare but fatal disease of the nervous system. Other reef organisms produce chemicals useful for cancer and AIDS research. Corals themselves produce a natural sunscreen, which chemists are developing to market in Australia, and their porous limestone skeletons are promising for bone grafts in humans.

For the 109 countries whose shores are lined with more than 100,000 kilometres of reefs, the ecosystem is a natural asset. Reefs provide immeasurable service by protecting coastal lands from erosion. Already, over two-thirds of the world's sandy shorelines are eroding. The results of losing reefs can be seen in Tanzania, where formerly protected resort beaches are eroding at a rate of five metres per year. The bill for restoring these self-repairing breakwaters can run to hundreds of millions of dollars. In the Maldives, for instance, the government destroyed a reef for a reclamation project: the replacement seawall cost $12 million for a little more than one kilometre.

Besides protecting coastlines, reefs also help form the idyllic white sand beaches and light turquoise lagoons that draw tourists to the tropics. In the Caribbean, the tourist Mecca, coastal tourism is worth over $7 billion annually. Other regions have seen coastal visitors double and triple in numbers over the last decade, and Southeast Asia has more then 110 existing and planned tourist sites along the coasts, many of which are reef-lined. Worldwide, coastal tourism is the largest sector of the $250 billion tourism industry, which is projected to be the number one industry by the turn of the century.

Locally, reefs are saltwater supermarkets of food and raw materials, especially for traditional coastal and island people. Pacific Islanders obtain up to 90% of their animal protein from reef fish, and people in Southeast Asia, the Caribbean and parts of south Asia and east Africa derive a substantial portion of the protein in their diets from the fish that live in these ecosystems.

Healthy reefs are thought to be among the most productive fisheries in the oceans - 10 to 100 times higher per unit area than the open ocean. The total catch from reefs is estimated at 4-8 million tons per year. This is about 10% of all the fish caught for use as human food. According to John McManus, a marine research scientist at the University of the Philippines Marine Science Institute in Bolinao, coral reefs may account for up to 20-25% of the fish catch of developing countries.

Small-scale fishers, who typically do not have the equipment to work the open ocean, rely heavily on the world's reefs for their livelihoods as well as their daily meals. Some four million small-scale fishers - about eight times as many people as work in commercial fishing and a third of all subsistence fishers - haul in their catch from coral reefs.

Beyond their considerable value as a natural resource, reefs have intrinsic worth as living, thriving, awe-inspiring ecosystems. Reefs are the largest structures built on the planet, unsurpassed even by human endeavours, such as the Great Wall of China.