The breathtaking reds, yellows, and purples of the Mesoamerican Reef have been turning sickly white, leading researchers on a desperate hunt to understand and fight the mysterious disease killing the Caribbean's corals.

In a little over a year, the Mexican Caribbean has lost more than 30 percent of its corals to a little-understood illness called SCTLD, or stony coral tissue loss disease, which causes them to calcify and die. It was first detected in the Mexican Caribbean in July 2018, near the reef's northern tip.

Experts warn the disease could kill a large part of the Mesoamerican Reef, a magnificent arc of more than 1,000 kilometers of coral shared by Mexico, Belize, Guatemala, and Honduras.

Scientists say SCTLD is even more dangerous than coral bleaching, which happens when environmental changes, such as warming ocean temperatures, cause corals to expel the microscopic algae that live inside them and give them their vibrant colors.

A reef can recover from coral bleaching if its environment recovers in time.

But SCTLD is fatal.

"It's a complete detachment of the coral tissue, which dies and leaves behind a white skeleton," said Claudia Padilla, a scientist at CRIP Puerto Morelos, a marine biology research center on Mexico's Yucatan peninsula.

Scientists are racing to understand what causes SCTLD.

One prime suspect is poor water quality, caused by sewage and a recent surge of decomposing sargassum seaweed.

Another likely factor is the chemicals in tourists' sunscreen, which the authorities have now banned.

Around 725,000 tourists have visited the Mexican Caribbean's reefs so far this year, a similar figure to previous years, according to officials.

(All photos via VCG.)

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