The Caspian Sea is technically the world's largest lake, but because it contains salt water and has a seafloor made of oceanic crust, it is often categorized separately. If you look for the largest purely freshwater lake by surface area, the answer is Lake Superior. Shared by the United States and Canada, it covers about 31,700 square miles. However, even this is a trick question: if you consider Lake Michigan and Lake Huron to be a single hydrological unit (since they are joined at the Straits of Mackinac and share the same water level), then Lake Michigan-Huron is actually the largest freshwater lake in the world by surface area. Most traditional maps list them separately, but geologists increasingly view them as one massive body of water.