Geography In Action
The lake was carved out by the Wisconsin ice sheet during the last ice age. At one point the lake was actually a bay on the Atlantic ocean. You can see dry beaches and banks up to 25 miles (ca. 40 km) from the current shores, tracing the changes to the lake over the millennia.
With the weight of the glaciers gone, the plates floating on the lava inside the globe are still bouncing back upwards. As the land rises out of the water, it tips a little, and inhabitants of the southern shores lose a bit of their land to the hungry ripples.