Because of their sea-like characteristics, such as rolling waves, sustained winds, strong currents, great depths, and distant horizons, the five Great Lakes have long been called inland seas.
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Fry, however, would not call the Great Lakes inland seas—at least not scientifically. Despite their size, the lakes are beholden to what happens on the land that surrounds them in a way larger seas are not.
For this reason, all seas are saline, or salty. Some seas are called bays (like the Bay of Bengal, between India, Bangladesh, Myanmar, and Indonesia), while some lakes are called seas (like the Caspian Sea, shared by Azerbaijan, Iran, Kazakhstan, Russia, and Turkmenistan). There are more than 50 seas on Earth.
And Caspian Sea is the largest lake in the world. But in the ancient time people noticed that it was salty and large, So they considered as sea. Both are salt lakes without any outlets. Still, They are continued to be called seas as they are too big to be called a lake.