Dainty blue fish dart around coral shaped like moose antlers near the Mexican resort of Cancun, but sickly brown spots are appearing where pollution threatens one of the world's largest reefs.
Parts of the reef, nestled in turquoise waters, have died and algae - which feed on sewage residues flowing out of the fast-growing resort city - has taken over.
Coral reefs like Chitales, near the northern tip of a Caribbean reef chain stretching from Mexico to Honduras, are dying around the world as people and cities put more stress on the environment.
Climate change alone could trigger a global coral die-off by 2100 because carbon emissions warm oceans and make them more acidic, according to a study published in December.
But local environmental problems like sewage, farm runoff and overfishing could kill off much of the world's reefs decades before global warming does, said Roberto Iglesias, a biologist from UNAM university's marine sciences station near Cancun.
"The net effect of pollution is as bad or maybe worse than the effects of global warming," said Dr Iglesias, a co-author of the study in the journal Science on how climate change affects reefs.
Human waste like that from Cancun's hotels and night spots aggravates threats to coral worldwide like overzealous fishing which hurts stocks of fish that eat reef-damaging algae. Coral reefs, underwater structures that look like rocky gardens, are covered with tiny animals called coral polyps.
The polyps build the reefs by slowly secreting calcium carbonate over thousands of years, creating structures that can dull the blow hurricanes deal to coastal cities and are vital nurseries for fish.
Across the Caribbean, the amount of reef surface covered by live coral has fallen about 80 per cent in the last three decades, the Global Coral Reef Monitoring Network says.
In the Pacific between Hawaii and Indonesia, reefs have been losing about 1 percent of their coral coverage annually over the last 25 years.
Some scuba diving instructors around Cancun are worried about the future of their trade. Jorge Olivieri, who has been taking tourists out diving in the area for the last 16 years, says some reefs are so damaged he would not take an experienced diver to see them.
In the late 1960s, Cancun was a barely inhabited strip of sand just off Mexico's Yucatan peninsula. Separated from the mainland by narrow straits on either end, just a handful of families tended coconut groves there.
Then Mexican bureaucrats, hungry for foreign currency and armed with statistics on sunshine, hatched a plan to turn the area into a tourist area.
Today, millions of people each year pack into hotels running the length of the strip, including American "spring breakers" drawn to bawdy bars and wet T-shirt contests.
The lagoon next to the hotel strip is murky and gives off a foul odor in parts. Only crocodiles swim there now.
Away from the lagoon, seawater samples from around Cancun show the levels of chemicals from human waste have increased steadily over the last decade, said Jorge Herrera, a marine biologist at the Cinvestav research center in the nearby city of Merida.
Cancun's waste treatment plants do not clean sewage enough to make it safe for coral, marine biologists say. The treatment plants kill bacteria that can be harmful to people but do not remove chemicals like phosphates.