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Vintage Maps From A Swashbuckling Era Are Leading Scientists To Forgotten Treasure: ‘Ghost’ Reefs

These intricate “Google Maps” for sailors are as tall as a grown man — and are helping today’s marine experts monitor our oceans.

Professor Loren McClenachan of Colby College was visiting the British Admiralty Library in Portsmouth, England, when she unearthed a series of magnificent antique nautical charts. Depicting the waters around the Florida Keys, the charts were made by British cartographers and dated from just before the American Revolution.

They were the works of the world’s best cartographers at the time; each chart was basically a Google Map for sailors: as tall as a grown man, sepia-colored, and packed with an incredible level of detail. “Really beautiful pieces of work,” said McClenachan.

Then and now: Key West, Florida. Image courtesy of the authors.

Among other things, the maps included notes on where to find fresh water, where to hunt sea turtles, and where to avoid dangerous — even potentially deadly — coral reefs.

It’s tempting to associate coral with pleasurable activities like scuba diving. But reefs have always been serious hazards for sailors. Ancient shipwrecks litter the waters around the Florida Keys, including the famous wreck of the Atocha, which lost $400 million in gold and jewels (not to mention all but five of its crew) when it hit a reef during a hurricane 1622. So when the British took the Keys in the French and Indian War in the 1760s, their mapmakers weren’t just making fanciful drawings of dragon-filled oceans. They were mapping out a plan for survival.

Nearly two and a half centuries later, McClenachan saw even more potential in the gorgeous old maps. They were a window into the past that could tell about the future. Since the maps were made, many of the coral reefs around South Florida have been lost. McClenachan wondered if the maps could help us rediscover, and learn from, those ancestral reefs.

This image shows Cheeca Rocks in the Florida Keys. Photo by The Ocean Agency/XL Catlin Seaview Survey/Christophe Bailhache.

Working with colleagues at The University of Queensland and the Australian Research Council, McClenachan and her team pored over the maps, using modern satellite photography to play “spot the difference.” What they found was astonishing: When compared to modern times, nearly half the reefs from the 18th century had disappeared. Closer to land, it was as much as 90%.

The specifics behind why the reefs had disappeared were beyond the scope of the study, but it’s likely that over the years, regular dredging (digging up the seabed to make it easier to ships to pass through) or near-shore development, such as sea walls, may have broken up the coral. Modern coral reefs still face these pressures, as well as climate change induced bleaching events that can kill off reef-making organisms.

Coral reefs are some of the most diverse ecosystems on Earth. Healthy reefs provide food, jobs, and tourism dollars to nations all around the world. The current reefs around the Florida Keys alone are estimated to be worth $7.6 billion.

By revealing the location of these forgotten pirate-era reefs, McClenachan’s work could help researchers understand long-term trends in coral cover and how it has been affected by human activity. This could, in turn, help us better understand both where we could potentially restore reefs and how to better protect animals like fish or sea turtles that depend on them.

The Upper Keys. Image courtesy of Loren McClenachan.

McClenachan is planning to return to the Admiralty Library early next year and believes this technique could be used to find more forgotten reefs around the world. Anywhere the British went, they mapped. Jamaica’s Kingston Harbor and Hong Kong Harbor, in particular, might have similarly detailed charts of similarly forgotten reefs.

George Gauld, the 18th-century mapmaker who created most of the charts McClenachan used, was not able to finish his work. He and his British colleagues were ultimately chased away by American privateers during the Revolutionary War. But their maps remain. And thanks to researchers like McClenachan, they’re still helping us understand the oceans we love to sail and which help our Earth function.

McClenachan’s work was published on Sept. 6 in the journal Science Advances.

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