1. Caspian Sea
It's called a sea, partly because of its size and its salt water. But geographically, because it's not connected to an ocean or sea and because it sits below sea level, the Caspian is considered a lake by most. It spans 152,000 square miles, nearly five times the size of Lake Superior, No. 2 on the list of largest lakes in the world. Five countries — Azerbaijan, Iran, Kazakhstan, Russia and Turkmenistan — surround the Caspian.
On the border between Europe and Asia, the Caspian is "endorheic" — there is no outflow from the lake. The Volga River, the longest river in Europe, supplies most of the Caspian Sea. In all, the Caspian contains somewhere around 40 percent of all the lake water in the world.