Susan Snow-Cotter (1961–2006) and Anthony Wilbur of the Massachusetts Office of Coastal Zone Management were instrumental in establishing and maintaining the cooperative seafloor-mapping program, which is funded by the Coastal and Marine Geology Program of the USGS and the Commonwealth of Massachusetts. Assistance in the field was provided by Emile Bergeron, William Danforth, Barry Irwin, and Charles Worley of the USGS. We thank Jane Denny, William Danforth, and David Foster of USGS for their help in processing the large amounts of acoustic data. Jennifer Martin arranged this document in HTML. This manuscript, maps, and GIS data benefited from reviews by Jane Denny, S. Jeffress Williams, Matthew Arsenault, Vee Ann Cross, Anthony Wilbur, and Daniel Sampson.