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 km 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 km and has an average depth of 30.48 m. Green Lake has 43.94 km of diverse shoreline, ranging from sandstone bluffs to marshes.