Battered by bleaching, Florida’s coral reefs are now facing a mysterious disease.

Brain corals in the Miami area have been developing white patches that progressively grow in size until they were completely covered. Four years later, scientists are still baffled by the mystery disease that is believed to affect about half of the Sunshine State’s coral species.

“It is just heartbreaking for us because it’s such an iconic reef. I can’t sleep at night because I think about it and what else can we do.” Biologist Erinn Muller, who leads coral health program at Mote Marine Laboratory, said.

In just four years, the so-far unidentified disease has already had a dramatic impact on Florida’s reef tract, which extends some 360 miles down the state’s Atlantic coast. Muller says it appears to be a bacterial disease, and for about half of the state’s species of coral it’s deadly.

“When they’re affected by this, the tissue sloughs off the skeleton,” she says. “And we see that once a coral is infected, it usually kills the entire coral, sometimes within weeks. And it doesn’t seem to stop.”

The disease appears to have affected brain and star coral species the most, which is crucial, as these species usually “form the foundation” for reef tracts.

Coral reefs are some of the most diverse ecosystems on Earth that harbor more than a quarter of all marine species. But these systems are under pressure from many factors including warming water, nutrient runoff, tourism and fishing. The mysterious coral destroying disease is the latest to join the list.