Marine scientists have long lamented the decline of Florida’s vibrant coral reefs. Now, nautical maps drawn just before the American Revolutionary War (similar to the one pictured) are giving scientists a clearer idea of just what has been lost. The maps, created by the British Admiralty between 1773 and 1775, record the location of coral reefs—which can be a deadly navigational hazard—along the Florida Keys in southern Florida. Scientists compared the maps to modern charts, finding that more than half of the historical reefs have been replaced by seagrass beds or bare sea floor. Most of the losses were in near-shore environments—approaching 90% in some areas—the team reports today in Science Advances. The map-sleuthing, the first of its kind, can’t pinpoint when the reefs disappeared. But it has highlighted areas where reefs once not only sheltered marine life, but helped protect the shore from hurricanes and other storms.