Why only Caspian surprised you? what about Black sea ?) 😜

There are many endorheic basins in the world. Parts of the Sahara are below sea level, but the rivers which flowed into them are extinct as the lakes. It's a sea because of its salinity, as salt isn't carried by evaporating water. Without evaporation from the Caspian Sea, the Hyrcanian Forest would die.