Which lake contains the world's largest island within a lake?

Lake Huron contains Manitoulin Island, the world's largest island in a lake.

Manitoulin Island lies in the northern part of Lake Huron in Ontario, Canada. It covers about 2,766 square kilometres and is itself divided by numerous inland lakes. Those smaller lakes make the record especially distinctive: an island in a lake contains lakes of its own.

Lake Huron is one of North America's five Great Lakes and is connected to Lake Michigan through the broad Straits of Mackinac. Hydrologically, the two lakes are often treated as a single system because their water levels are nearly the same.

The record concerns an island located within a lake, not the largest island in the world or the largest island in a sea. Greenland holds the global island-size record, while islands such as Isle Royale and Beaver Island are notable Great Lakes islands but are much smaller than Manitoulin.