Maple Leaf, a side-paddlewheel freight and passenger steamer launched in 1851 in Kingston, Upper Canada, later served as a Union Army transport during the Civil War. On April 1, 1864, while crossing the St. Johns River near Jacksonville, Florida, she struck a Confederate mine, then called a torpedo, and sank with the loss of four crewmen in the war’s first mine casualty. Although parts above the main deck were later removed to protect navigation, the lower wreck survived unusually well. Rediscovered in 1984, it became an exceptionally important archaeological site for preserving both a Union transport’s wartime contents and a rare mid-19th-century Great Lakes steamer.