The Caspian Sea
The Caspian Sea is the largest lake in the world. It is called the sea because it is so big and because the water contains quite a lot of salt. It is about a third as salty as sea water. Nevertheless, the Caspian Sea is a lake because it is surrounded by land. However, you can travel via rivers and canals to the Black Sea and the Baltic Sea, which are both "real" seas.
The surface area of the Caspian Sea is over 370,000 square kilometres. That makes it slightly larger than Germany. If you add up all the water that exists in the world in lakes, almost half of it is in the Caspian Sea.
In former times, the Caspian Sea was inhabited by the Caspian people, from whom it takes its name. At least that is what the ancient Greeks claimed. In other languages, the lake has other names. The Iranians, for example, say Lake of Māzandarān, after an ancient area in the south of the lake.
For a long time, there was only one other country besides Iran that had a coast on the Caspian Sea: the Soviet Union. In 1991, it broke apart. It now became Russia, Kazakhstan, Turkmenistan and Azerbaijan. In Azerbaijan, on the south of the lake, a lot of oil has been found.
Most of the water that flows into the lake comes from the Volga. It is the longest river in Europe. In the past, a lot of water was taken from the tributaries. That was one of the reasons why the lake became smaller. In the meantime, however, the water level has risen again.