Grand Lake St. Marys is an artificial lake located just west of St. Marys, Ohio. The lake covers 13,500 acres (55 km²) in Auglaize and Mercer counties. Grand Lake St. Marys is the largest inland lake in Ohio in terms of land area, but is extremely shallow, with an average depth of only 5 to 7 feet (1.5 to 2 m). It was constructed in the early 1800s as a reservoir for the Miami and Erie Canal. The lake and the surrounding state park are a popular site for recreational fishing and camping.