Caspian Sea: The World's Largest Lake (Yes, You Read That Right)

By: Marie Look  | 
Map of Caspian Sea, focus on Baku, Azerbaijan, and the surrounding area
This map of the Caspian Sea region shows that the body of water is landlocked. 200mm / Getty Images

One of Earth's most interesting natural features, the Caspian Sea is the largest inland body of water on the planet, defying conventional classifications to be both a sea and a lake.

Situated between Europe and Asia, this colossal sea has plenty of other unique qualities too, such as its unusual salinity, distinct geography and biodiversity, and its economic and political significance to the five nations that surround it.


Geography of the Caspian Sea

Nestled within the heart of the Caspian region, the Caspian Sea lies at the crossroads of continents, bordered by five countries: Kazakhstan to the northeast, Turkmenistan to the east, Iran to the south, Azerbaijan to the west and Russia to the northwest.

Stretching over 745 miles (1,200 km) from north to south and spanning a surface area of approximately 143,200 square miles (371,000 square km), the Caspian Sea holds the title of the world's largest inland body of water.


For reference, this means its surface area is roughly equal to the country of Japan.

The Caspian Sea basin, encompassing a vast catchment area, is fed by numerous rivers, the most notable being the Volga River, which empties into the northern Caspian. Smaller rivers such as the Ural River contribute to the inflow as well, enriching the sea with freshwater inputs.


Formation of the Caspian Sea

The formation of the Caspian Sea unfolded over millions of years. Unlike the oceans, this body of water is not the result of tectonic plate movement or continental drift. Instead, it emerged through a combination of tectonic activity, geological processes and climatic changes.

During the late Cretaceous period, around 70 to 60 million years ago, the region that now encompasses the Caspian Sea was part of the ancient Tethys Sea, a vast ocean separating the supercontinents of Laurasia and Gondwana.


Over time, as tectonic plates shifted and collided, the Tethys Sea began to fragment and shrink, giving rise to smaller basins and seas, including the Caspian Sea region.

The gradual convergence of Eurasian and Arabian tectonic plates led to the uplift of mountain ranges, including the Caucasus Mountains to the west and the Alborz Mountains to its south. These mountain ranges acted as natural barriers, trapping water and creating a basin that eventually became the Caspian Sea.

At the same time, Earth's climate was fluctuating over millions of years, causing periods of glaciation and global warming to alter the sea level and precipitation patterns. During glacial periods, large ice sheets formed, locking up water and causing global sea levels to drop. This caused the Caspian basin to be cut off from the oceans reduced the volume of inflow from rivers.

During interglacial warming periods, melting ice sheets and increased rainfall led to rising sea levels and greater inflow of freshwater from rivers. These fluctuations in sea level and freshwater input affected the Caspian Sea's size, depth and salinity over time.


Topographical Characteristics

What sets the Caspian Sea apart is not just its size, but also its distinct features. If you divide the sea into three main sections — the northern Caspian Sea, middle and southern Caspian — a diverse topography reveals itself.

  • The northern Caspian, characterized by its shallow depths, gently slopes towards the south, where it meets the middle Caspian.
  • In the middle, the sea becomes deeper compared to the north Caspian, with an average depth of about 623 feet (190 meters) and a maximum depth of more than 3,280 feet (1,000 meters), or about two-thirds of a mile (1 km).
  • The abrupt western slope contrasts sharply with the gentler eastern gradient.
  • The southern Caspian, nestled along the Iranian shore, presents a different landscape altogether. Here, the sea meets the towering peaks of the Caucasus Mountains, creating a narrow marine plain before plunging into deeper waters.

The Caspian Sea's southern coastline is dotted with the coastal towns of Northern Iran, each with its own unique charm. Among the most prominent is Bandar-e Anzali, which has been a major port city on the Caspian Sea since the early 19th century.


Unusual Salinity

The Caspian Sea is also unique for its unusual salinity, compared to typical oceans and seas. Essentially, it displays the characteristics of both freshwater and saltwater bodies.

Unlike most seas, which have open connections to the ocean and experience regular tidal influxes of saltwater, the Caspian Sea is largely landlocked. This limited exchange with oceanic waters restricts the inflow of saltwater into the Caspian basin.


On the other hand, the Caspian Sea receives significant freshwater inputs from various rivers, including the Volga, Ural and Kura. These rivers carry freshwater from distant mountain ranges and vast plains, diluting the salinity of the Caspian Sea. The Volga River, in particular, is the largest contributor of freshwater to the Caspian, accounting for the majority of its inflow.

Despite the influx of this freshwater, the Caspian Sea does not exhibit the low salinity levels characteristic of most freshwater lakes.

Instead, it maintains a moderate salinity level, averaging around 12 parts per thousand (ppt). This makes the Caspian sea water saltier than freshwater (with a typical salinity of 0.5 ppt) but less salty than the ocean, which has an average salinity of 35 ppt.


Biodiversity and Ecology

The intermediate salinity levels of the Caspian Sea make it a good habitat for species that have adapted to brackish water conditions.

Beneath the surface of the Caspian Sea are an array of fish species, including the Caspian sturgeon, whose roe yields the coveted delicacy known as caviar. Other inhabitants include the Caspian seal, the only aquatic mammal native to the sea, and the Caspian gull and Caspian tern, which find sanctuary along the shores.


Unfortunately, the Caspian Sea's biodiversity faces numerous threats, ranging from overfishing to habitat degradation. The exploitation of energy resources, such as oil and natural gas, has led to environmental concerns, impacting both marine life and coastal ecosystems.

Conservation efforts led by organizations such as the United Nations Environment Programme strive to protect the Caspian's delicate balance and ensure the sustainability of its resources for future generations.


Geopolitical Dynamics

The geopolitical borders of the Caspian Sea have been a source of contention for decades among the Caspian littoral states (the countries along the sea's shores): Kazakhstan, Turkmenistan, Iran, Azerbaijan and Russia. Disputes over territorial waters and the delineation of exclusive economic zones are ongoing.

These five countries, each with its own interests and agendas, must regularly navigate complex negotiations over resource extraction, maritime boundaries and environmental conservation as in relation to the world's largest inland body of water.


For example, Iranian patrol boats cruise the south Caspian, asserting Iran's sovereignty over its share of the sea, while Russia maintains a strong presence in the northern Caspian. Naval forces from all five countries maintain a presence on and around the sea.

Economic Significance

The Caspian region holds immense economic significance due to its abundant natural resources and strategic location. The countries along the Caspian coastline have invested heavily in capturing these energy resources, leveraging their maritime assets to bolster their economies.

The body of water is renowned for its vast reserves of oil and natural gas, attracting international oil companies and spurring the growth of a thriving energy industry. As a result, offshore oil platforms for drilling dot the horizon in the area.


The Caspian Sea also supports a thriving fishing industry, providing livelihoods for coastal communities. Sturgeon fishing, in particular, has been a traditional practice for centuries.

Additionally, the body of water serves as a crucial maritime artery for trade and transportation, linking Central Asia to global markets and providing access to international shipping routes. Coastal tourism and aquaculture industries have also been important to the region's prosperity.

Sustainable management practices and conservation initiatives will continue to be crucial for safeguarding the Caspian's valuable resources. This will likely require unprecedented collaboration among the Caspian littoral states as well as international stakeholders.

This article was created in conjunction with AI technology, then was fact-checked and edited by a HowStuffWorks editor.