Grand Lake St. Marys is a 13,500-acre lake in Mercer and Auglaize counties, Ohio. It is the largest inland lake in Ohio in terms of area, but is shallow, with an average depth of only 5–7 feet.

Grand Lake St. Marys Seaplane Base (FAA LID: O12) is a public use seaplane base located five nautical miles (9 km) southwest of the central business district of St. Marys, a city in Auglaize County, Ohio, United States. It is owned by the Ohio Division of Parks & Recreation and is located on Grand Lake St. Marys, a lake and state park in Ohio. The airport is a significant meeting place for the Ohio Seaplane Pilots Association.

Grand Lake St. Marys Seaplane Base covers an area of 643 acres at an elevation of 871 feet (265 m) above mean sea level. It has one seaplane landing area designated 9/27 and measuring 7,000 x 4,000 ft (2,134 x 1,219 m). 

History

Grand Lake St. Marys was constructed in the early 19th century as a reservoir for the Miami and Erie Canal, which connected the Ohio River with Lake Erie. At one time, it was the world’s largest man-made lake. The canal system thrived for about thirty years in the mid-to-late 19th century before it was replaced by the railroads. In the 1890s, oil was discovered in the area, and oil derricks were set up in the lake around 1891, making Grand Lake the location for the first offshore oil drilling to take place.