The largest lake by surface area in the world is the Caspian Sea. Despite its name, it is classified as a lake due to its landlocked nature. It has a surface area of approximately 371,000 square kilometers (143,000 square miles). The Caspian Sea is bordered by five countries: Russia, Kazakhstan, Turkmenistan, Iran, and Azerbaijan. Its unique ecology and significant natural resources, including oil and gas reserves, make it a vital geopolitical region.