Mallows Bay-Potomac River National Marine Sanctuary encompasses 18 square miles of waters and submerged lands within the tidal Potomac River in Maryland state waters. It protects nationally significant maritime cultural heritage resources, including the historic remnants of World War I-era wooden steamships known as the Ghost Fleet. The sanctuary also includes Native American sites, remains of historical fisheries operations, and Revolutionary and Civil War battlescapes. The area is listed in the National Register of Historic Places. The Ghost Fleet ships were built as part of the United States' engagements in World War I. Although the ships never saw battle, their construction at more than 40 shipyards in 17 states reflects the massive national wartime efforts that drove the expansion and economic development of communities. Today, nature has reclaimed the ships, and the overgrown wrecks provide habitat for fish, beavers, and a variety of birds, including osprey, bald eagles, and blue heron.