The Caspian is the largest enclosed body of water on Earth: roughly 371,000 square kilometres, larger than Germany, with no outlet to any ocean. Its surface sits about 28 metres below global sea level. Its water is brackish rather than salty — around a third of ocean salinity — because the Volga and other northern rivers pour in enormous volumes of fresh water at one end while the arid south evaporates it away at the other.
Whether it is a sea or a lake sounds like a question for pedants. It is in fact the question that five governments spent most of three decades arguing about, because the answer determines who owns the oil underneath it.
A Remnant of a Vanished Ocean
Geologically the Caspian is what remains of the Paratethys, an ancient sea that once stretched across much of Eurasia and was cut off from the world ocean millions of years ago. That isolation is why the basin's biology is so peculiar: a large share of what lives in it lives nowhere else.
The clearest example is the Caspian seal, the only seal species on Earth confined to an inland body of water. Estimates put its population at around a million at the start of the twentieth century; it is now assessed as endangered, with a fraction of that number remaining, hit by a century of commercial hunting, habitat loss on the northern ice where the pups are born, and periodic mass mortality events.
Then there are the sturgeon — beluga, ossetra, sevruga — prehistoric-looking fish that can outlive the people who catch them. For most of the twentieth century this basin produced the overwhelming majority of the world's wild caviar. Overfishing and poaching after the Soviet collapse gutted the stocks, wild catch for export is now tightly restricted under international agreement, and the caviar sold legally by Caspian states today comes overwhelmingly from aquaculture rather than the sea itself.
Sea or Lake: Why It Mattered
Under international law the distinction is consequential. Treated as a sea, the Caspian would fall under the Law of the Sea, giving each state a territorial belt and an exclusive economic zone. Treated as a lake, it would by convention be divided into national sectors by agreement among the shoreline states — or held in common.
Until 1991 the question barely arose: the shoreline was shared by two countries, the Soviet Union and Iran. The Soviet collapse turned two claimants into five — Azerbaijan, Russia, Kazakhstan, Turkmenistan and Iran — with substantial oil and gas at stake and no agreed method of division.
The framework Convention on the Legal Status of the Caspian Sea was finally signed at Aktau in 2018. It splits the difference: the surface water is treated largely as common space with freedom of navigation, while the seabed and its resources are divided into national sectors by agreement between neighbours. Several of those seabed boundaries had already been settled bilaterally — Azerbaijan with Kazakhstan and Russia around the turn of the century — and Azerbaijan and Turkmenistan reached an understanding on a long-disputed offshore field in 2021. Others remain open, and Iran, whose share under most proposed formulas is the smallest, has continued to press a different reading.
The Water Is Going Down
The more pressing problem now is volume. The Caspian's level fluctuates naturally over decades, but it has been falling since the mid-1990s, and the decline has accelerated. Reduced inflow from the Volga combined with higher evaporation in a warming climate has already exposed large areas of the shallow northern shelf, and published projections for the rest of this century range from several metres of further decline to considerably more under high-emissions scenarios.
The consequences are not evenly distributed. The northern Caspian, where the water is only a few metres deep, is the part that disappears first — and it is also the seal pupping ground and the sturgeon nursery. Ports are affected too: Aktau on the Kazakh side has had to contend with falling depths on its approaches, which matters for a crossing that exists to move freight.
Seeing It From the Water
For all its size, the Caspian is surprisingly hard to actually get onto as a visitor. There is no cruise industry, the ferry between Kazakhstan and Azerbaijan is a freight service that tolerates passengers rather than courting them, and most of the coastline on every side is industrial, military or simply empty.
Baku is the exception. Short sightseeing sailings run out of the bay through the afternoon and evening, and they are the most accessible way for anyone without a freight booking to spend time on the water — Nargin Island, the tankers waiting at anchor, and the oil-era city rising behind them all read very differently from a deck than from the promenade. Tourister's guided Baku boat tour covers the practical side of arranging one.
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