There are over 304 million lakes on the planet, but one above all dominates in size so overwhelmingly that it seems almost incredible: the Caspian Sea. With a surface area of ​​up to 436,000 square kilometers, this immense body of water is even larger than the whole of Japan, and almost five times larger than Lake Superior, between Canada and the United States.
Sea or lake? A question still open
The Caspian Sea is in a category all its own. It is the largest landlocked body of water in the world, and for this reason it is considered both the largest lake on Earth and one of the smallest seas. In fact, it has an extension, depth and salinity greater than that of a lake, but being completely closed, it can also be classified as such.
The distinction is not just semantic: calling it a sea or a lake has a direct impact on international legislation, the exploitation of natural resources and the policies between the countries bordering it. This is a question that is still partially unresolved.
A lake that is also a border between continents
The Caspian extends between Europe and Asia, nestled east of the Caucasus Mountains and west of the great Central Asian steppe. Its waters touch the borders of five countries: Kazakhstan, Russia, Azerbaijan, Iran and Turkmenistan. Its surface is located approximately 27-28 meters below sea level, while the average depth is around 200 meters, with peaks exceeding 1,000 meters below sea level. The overall river basin extends for approximately 3.5 million square kilometers.
With an age of approximately 5.5 million years in its current form, the Caspian has much older geological origins: its evolution dates back to the late Miocene, approximately 13.8 million years ago, when it was still connected to the Black Sea through a depression now known as the Manych Trench. A tectonic uplift then made it a closed basin, forever sealing its geological history — and beginning its extraordinary uniqueness.
Why are its waters salty?
Being an endorheic basin, i.e. without emissaries, the Caspian has accumulated over time the salts brought by the rivers and those deriving from the evaporation of the Mediterranean waters with which it was anciently connected. The average salinity is around 13 grams per liter, about a third of that of the oceans, but with a chemical composition different from sea water: high concentration of sulphates, calcium and magnesium carbonate, and low chloride content, the result of millennia of evaporation and run-off alternating with glacial and post-glacial periods.
Fresh water in the middle of Asia
Over 130 rivers feed the Caspian, releasing an average of 300 cubic kilometers of fresh water every year. The main contribution comes from the Volga, the longest river in Europe, which alone provides 80% of the total water supply. The Ural, Kura, Kuma and Terek also contribute to the hydrological balance of this water giant.
Important cities are located along its banks: Baku, the capital of Azerbaijan, is the largest metropolis overlooking the Caspian, followed by Astara in Iran, Atyrau in Kazakhstan and Derbent in Russia. In its waters there are also around fifty small islands, mainly concentrated in the northern coastal areas.
It is also worth remembering that the subsoil of the Caspian is very rich in hydrocarbons: in this area are the deposits from which the TAP gas pipeline starts, which allows Italy to import natural gas from Azerbaijan.
An extraordinary ecosystem, but increasingly fragile
The Caspian is home to approximately 850 animal species and over 500 plant species, many of which are endemic and not found elsewhere on the planet. Among its most famous inhabitants are six species of sturgeon in critical danger of extinction – from which the prized caviar comes -, the Caspian seal, a unique mammal of its kind in this part of the world, and the beluga.
However, the lake ecosystem is under increasing pressure. The pollution produced by the oil industry, plastics, nutrients brought by rivers and dams on tributary waterways are altering millenary balances. Added to all this is the climate crisis: according to the most recent estimates, based on altimetric data collected by NASA from the Global Water Monitor, the water level of the Caspian could drop from 9 to 18 meters by the end of the century. This would mean losing around a quarter of the lake surface and discovering over 93,000 square kilometers of seabed, an area equivalent to the whole of Portugal.
A water giant that the world should learn to know and, above all, protect before it is too late.
