Geologically an ocean The most complete citation regarding the formation of the Caspian Sea and whether it s a lake or something else is The paper published in American Society of Limnology and Oceanography gives a detailed explanation of how the Caspian Sea is essentially 3 lakes of which the northern two were not oceanic and it concludes Conclusion A lake not a sea The Caspian is truly unique among aquatic ecosystems not only by its volume but first and foremost by its longitudinal zones of salinity depth and climate These three zones combine independently to produce three basins that are more different from each other than are many individual lakes The dimictic north behaves like a temperate shallow lake The central basin is still largely dimictic The south but for a delicate salinity driven mechanism of reoxygenation would rapidly revert to a meromictic basin comparable to the Black Sea Here however meromixis only set in at the start of the Holocene when the then Pontic Lake became united with the Mediterranean Sea and acquired a number of typical marine traits including many truly marine biota But the biota of the Caspian basins are all euryhaline and typical marine groups are lacking Furthermore intralacus trine speciation within that euryhaline gene pool produced a level of endemism comparable to that of other ancient lakes e g Lake Baikal suggesting similar mechanisms of spe ciation I conclude that except for its sheer size there are no arguments except political ones to call the Caspian a sea Not only has it never been connected to the world ocean but even when united with the Black Sea that was when the Pontic basin still contained a true lake itself Obviously this is one published viewpoint and there may be others but it certainly means we simply say the Caspian is geologically an ocean obviously the author would include ocean as he says Not only has it never been connected to the world ocean