Lake Manitou

Lake Manitou is the largest lake of the 108 lakes on Manitoulin Island with a surface area of approximately 104 sq. km or 40.5 sq. miles. It also can lay claim to being the largest lake on a freshwater island in the world.
The Lake Manitou Area Association (LMAA) was established in 1993 and currently has 231 members.
Lake Manitou is the largest lake on Manitoulin Island in Ontario, Canada. It is the largest lake on a lake island in the world.
Since Manitoulin Island itself is in Lake Huron, one of the Great Lakes, Manitou qualifies as the largest "lake in a lake". Lake Manitou has an area of about 104 square kilometres