Okefenokee National Wildlife Refuge (NWR) was established in 1936 to preserve the 438,000 acre Okefenokee Swamp. Presently, the refuge encompasses approximately 402,000 acres. In 1974, to further ensure the protection of this unique ecosystem, the interior 353,981 acres of the refuge were designated a National Wilderness Area. The swamp remains one of the oldest and most well preserved freshwater areas in America and extends 38 miles north to south and 25 miles east to west. Okefenokee is a vast bog inside a huge, saucer-shaped depression that was once part of the ocean floor. The swamp now lies 103 to 128 feet above mean sea level. Native Americans named the area "Okefenokee" meaning "Land of the Trembling Earth". Peat deposits, up to 15 feet thick, cover much of the swamp floor. These deposits are so unstable in spots that trees and surrounding bushes tremble by stomping the surface.