Green Lake — also known as Big Green Lake — is a lake in Green Lake County, Wisconsin, United States. Green Lake has a maximum depth of 237 ft, making it the deepest natural inland lake in Wisconsin and the second largest by volume. The lake covers 29.72 km² and has an average depth of 30.48 m. Green Lake has 43.94 km of diverse shoreline, ran…Green Lake — also known as Big Green Lake — is a lake in Green Lake County, Wisconsin, United States. Green Lake has a maximum depth of 237 ft, making it the deepest natural inland lake in Wisconsin and the second largest by volume. The lake covers 29.72 km² and has an average depth of 30.48 m. Green Lake has 43.94 km of diverse shoreline, ranging from sandstone bluffs to marshes.