This data set represents the extent and approximate location of historic wetland habitats in certain areas of the conterminous United States. The identification of these historic wetlands is limited by the methodology outlined in the associated Historic Wetlands Project Metadata and is not intended to be a comprehensive inventory of all historic wetlands. The U.S. Fish and Wildlife Service (Service) is the principal Federal agency that provides information to the public on the extent and status of the Nation's wetlands and provides stewardship for the wetlands data that comprise the Wetlands Layer of the National Spatial Data Infrastructure. In the formulation of this data layer, historic wetlands are defined as areas where there is evidence that a wetland once existed. This evidence can be from historical map information inventories of past wetland extent or other information collected that relate directly to data on wetland filling, drainage or other modifications. Historic wetlands have been identified using several different techniques depending on the availability and type of information used to locate these areas and user needs. For example, historical maps often provide information about past wetland extent or location and can be useful tools to identify historic wetlands. Historic wetlands have been identified as polygonal data. No linear features have been included. Historic wetland polygons are not classified as wetlands and have no wetland labels or attribution. Since these features no longer exist, boundary delineations are considered approximations based on topography, previously mapped information or indications of historic water levels.