In 1775, George Gauld, a surveyor for the British Admiralty, charted the waters off the coast of the British colony of West Florida.  Recently, Loren McClenachan, historical ecologist and professor of environmental studies at Colby College, has compared Gauld’s charts of the coral reefs along the Florida coast to modern imagery. The results were grim. Compared to Gauld’s charts, far more coral reefs have disappeared than had been previously thought. The reefs have become ghosts.

Gizmodo reports: By comparing Gauld’s maps with modern coral cover information from several databases, McClenachan and her colleagues arrived at a bleak conclusion: roughly half of the seafloor occupied by corals in the vicinity of the Florida Keys in the late 18th century no longer is. Much of the dieback seems to have occurred in Florida Bay (where coral cover was an estimated 88% higher in the late 18th century) and close to shorelines (an estimated 69% higher per Gauld’s maps).

Importantly, this suggests recent estimates of coral dieback in the Keys, derived from data collected in the 1970s, ‘80s, and ‘90s, grossly underestimate the degree to which these ecosystems have receded, according to the research published this week in Science Advances.

“We’ve really lost these nearshore reefs—lots of them probably disappeared before we even started studying them in the water,” McClenachan said. She suspects the dieoff patterns are due to greater human disturbance close to shorelines, and in the case of Florida Bay, changes following the widespread drainage of the Everglades in the early 20th century.