Washington ? A one-two punch of bleaching from record hot water followed by disease has killed ancient and delicate coral in the biggest loss of reefs scientists have ever seen in Caribbean waters.

Researchers around the globe are scrambling to figure out the extent of the loss. Early conservative estimates from Puerto Rico and the U.S. Virgin Islands find that about one-third of the coral in official monitoring sites recently has died.

“It’s an unprecedented die-off,” said National Park Service fisheries biologist Jeff Miller, who last week checked 40 stations in the Virgin Islands. “The mortality that we’re seeing now is of the extremely slow-growing reef-building corals. These are corals that are the foundation of the reef. : We’re talking colonies that were here when Columbus came by have died in the past three to four months.”

Some of the devastated coral can never be replaced because it only grows the width of one dime a year, Miller said.

Coral reefs are the basis for a multibillion-dollar tourism and commercial fishing economy in the Caribbean. Key fish species use coral as habitat and feeding grounds. Reefs limit the damage from hurricanes and tsunamis. More recently they are being touted as possible sources for new medicines.

If coral reefs die “you lose the goose with golden eggs” that are key parts of small island economies, said Edwin Hernandez-Delgado, a University of Puerto Rico biology researcher.

And with global warming, scientists are pessimistic about the future of coral reefs.

“The prognosis is not good,” said biochemistry professor M. James Crabbe of the University of Luton near London.