The body of water that is the largest lake in the world is Caspiann Sea, which, despite its name, is technically a lake and not an ocean. With a surface area of approximately 371,000 square kilometers (143,000 square miles), it borders five countries: Russia, Kazakhstan, Turkmenistan, Iran, and Azerbaijan. The Caspian Sea is a unique ecosystem, home to various marine species, including the famous beluga sturgeon, and is also rich in natural resources like oil and gas.