The largest lake by surface area in the world is the Caspian Sea, despite its name. It stretches over 371,000 square kilometers and is bordered by five countries: Russia, Kazakhstan, Turkmenistan, Iran, and Azerbaijan. The Caspian Sea is unique as it is a saltwater lake, and it contains a diverse range of ecosystems. It has been historically significant for trade, fishing, and oil extraction, illustrating its importance in both economic and ecological contexts.