Thailand is closing dozens of dive sites to tourists after unusually warm seas caused severe damage to coral reefs in the Andaman Sea, one of the world’s top diving and beach resort regions, authorities said on Thursday.

More than half of southern Thailand’s 15,000 hectares of coral reefs are suffering from bleaching, or the shedding of coral colors, a phenomenon caused largely by rising sea temperatures over an extended period, officials said.

“We will study the cause and effect and find a way to restore them,” Sunan Arunnopparat, director of the Department of National Parks, told Reuters, adding that the reefs will be closed across seven national parks.