The National Wetland Inventory (NWI) is a comprehensive geospatial database maintained by the United States Fish and Wildlife Service (USFWS) that provides detailed information on the location, extent, type, and condition of wetlands across the United States. The Wetlands Data Layer is the product of over 45 years of work by the National Wetlands Inventory (NWI) and its collaborators and currently contains more than 35 million wetland and deepwater features. Massachusetts wetland areas have been updated between 1975 and 2013, depending on the part of the state. You can learn more about NWI data here.