Scientists have used detailed nautical maps created by British sailors in the 1700s to study more than two centuries of coral loss in the Florida Keys. They found that over the past 240 years, the region has lost more than half of its coral structures, with some areas, particularly closer to shore, either gone completely or having lost up to 90 percent of their extent.

The researchers published their findings this week in the journal Science Advances.

“We found that reef used to exist in areas that today are not even classified as reef habitat anymore,” co-author John Pandolfi, a palaeoecologist at the University of Queensland, said in a statement.

Cartographers for the British Royal Navy created the detailed historical maps of the Florida Keys in the late 18th century for navigational purposes. They contained “substantial amounts of ecological information,” the study says, “with coral of particular interest as a navigational hazard.”