More than 240 years ago, the mapmaker George Gauld put pen to paper and drew the Florida Keys. The scant frill of islands curls across the ocean, surrounded by minute notations of depth—and surprisingly detailed descriptions. “A great part of this extensive Bank is quite dry at low water,” reads one notation. “This bank is full of Coral Patches,” says another.

Gauld’s intention was probably primarily to highlight “hazards to navigation,” says the historical ecologist Loren McClenachan. “But he also happened to have a natural-history bent. He included information he didn’t necessarily have to. He drew the mangroves, and he wrote ‘seagrass on the bottom,’ and he wrote, ‘this is where the turtles nest.’”

In particular, Gauld’s meticulous recording of the coral reefs surrounding the Keys has enabled McClenachan and collaborators to compare coral cover in 1775 with the present day—a technical feat that reveals just how much the human presence has shaped the area’s undersea ecology.

McClenachan, a professor of environmental studies at Colby College, first came upon the charts about ten years ago, on a hunt through historical archives. She put the map of the Lower Keys up as art: “I was living in this tiny graduate-school apartment, and it covered most of the wall,” she recalls. Eventually, she began to search for modern data sets that covered the same area and contained the same kind of detailed information. While a fair bit is known about reefs from very long ago, thanks to paleoclimate researchers, and the recent catastrophic declines of coral cover around the world are quite clear, data about where things stood a few hundred years ago is harder to come by. But it’s a record of a time when humans first began to exert a particularly strong influence on the environment.

By merging three recent data sets—from the National Oceanic and Atmospheric Administration and other sources—McClenachan and her collaborators were able to pull together information covering the same region. Then, they matched up the coral on Gauld’s map with the modern reefs. “Some of the reef aligned just about perfectly,” McClenachan says. Since reefs are constructed over thousands of years, in the absence of direct disturbances, they will persist in the same locations for a very long time. Gauld was meticulous in distinguishing between rocks on the seabed and coral, along with many other features, and the researchers had reason to believe in the accuracy of his depiction.

If there was no coral within three-fourths of a kilometer of Gauld’s markings, the researchers declared the patch gone. Many of the reefs, unfortunately, had completely disappeared. More than 50 percent of the area that used to be covered with coral in the Keys was no longer. Corals close to the shore and in the Florida Bay were especially hard-hit. In the Bay, where sediments accumulated after the draining of the Everglades in the early 20th century and the construction of a railway, coral cover declined by 87.5 percent. Close to the shore, where anchors, dredging, and reclamation have affected the sea bottom, it declined by 68.8 percent.

The comparison made it clear that if we are interested in restoring coral reefs to their previous extent, we might have farther to go in getting back to historical levels than estimates based on more recent snapshots.

Still, says McClenachan, the Keys have been leaders in the area of coral restoration, a process of growing corals in aquariums and then planting them in the wild. “Maybe some of these places that used to have reef would be good to have reef again,” she says. Understanding where coral used to thrive might help us see where, with the right preparations, they could live again.

The details Gauld’s chart captured, with its sand banks, mangroves, turtle nests, and coral patches, could easily have gone unnoticed, and today our knowledge of the Keys would be poorer. We have this information “just because because this particular chart maker was paying attention,” McClenachan says. Today, in the face of great challenges, modern ecologists persevere in documenting and studying the great dying of coral reefs—taking notes, taking images, making maps.

“I think it’s necessary work; I don't think it’s necessarily uplifting work,” says McClenachan. “I am hopeful that it’s useful.”