Some coral reefs around Christmas Island have gone from vibrant ecosystems to graveyards in a mere five years — all because of a mysterious syndrome that scientists know next to nothing about, reports Elizabeth Preston for Hakai magazine.

The disease, which scientists call “white syndrome,” kills coral tissue and leaves a white calcium carbonate skeleton behind. (This is different from coral bleaching, which also turns coral white, but doesn’t directly kill the organism.)

For some reason, white syndrome only affects the Acropora genus — corals that form the backbone of many reefs in the Indian and Pacific oceans. Exactly how the condition spreads and infects corals remains somewhat enigmatic. Though scientists have linked white syndrome to things like pathogenic microbes, parasitic worms and rising ocean temperatures, Preston notes that the term is really used as a catchall for uncertain causes that produce the same effect.

Scientists first noticed white syndrome in the Great Barrier Reef in 1998. Since then, it’s popped up in the Caribbean and the Red Sea. The Christmas Island outbreak marks the first time white syndrome has been spotted in the Indian Ocean, explains Preston. A recent analysis of the outbreak reveals its quick and lethal spread from 2008 to 2013. At some reef sites around the island, the disease has killed 96 percent of local corals.

Preston notes that things like climate change and ocean acidification present a bigger threat to corals than white syndrome. But that doesn't change the danger of the enigmatic disease — after all, gorgeously fragile reefs protect both wildlife and people from harm.