The answer is Lake Michigan.
Lake Michigan is 22,316 sq miles in size and is the largest freshwater lake (totally within) the United States. It is 307 miles (494 km) long and 118 miles (190 km) at its widest point. During the last Ice Age, North America was sculpted by massive glaciers resulting in craters formed by retreating (melting) ice. During that process the Great Lakes were born and the state of Michigan took shape.