The Okefenokee Swamp, on the border of Georgia and Florida, is the largest intact freshwater swamp in the US. Its slow-moving blackwaters are stained by tannins from decaying organic matter and this vast ecosystem, covering 700 square miles, is packed with wildlife. Herons, cranes, bitterns, American alligators, turtles, snakes, frogs, and Florida black bears are among creatures who roam the swamp, which is largely protected by the Okefenokee National Wildlife Refuge. In the early years of the 20th century, part of the swamp was logged, and its peat deposits were mined for use as fuel. Remnants of railroad lines can still be seen crossing swamp waterways as well as other equipment dating from its time as a logging area.