The Caspian Sea sits in a depression on the boundary between Asia and Europe, bordered by five countries — Russia, Kazakhstan, Turkmenistan, Iran, and Azerbaijan — and surrounded entirely by land. It is, by surface area, the largest enclosed body of water on Earth: 371,000 square kilometres, about the size of Japan. Its waters are slightly salty in some parts and nearly fresh in others. It has no outlet to any ocean. It contains enormous reserves of oil and gas. It supports a fishery that produces a substantial fraction of the world's caviar. And for about three decades after the Soviet Union's 1991 collapse, the five surrounding states could not agree on whether it was legally a lake or a sea, with profound implications for who owned what. The question is now formally settled, but the underlying ambiguity reflects something deeper: the Caspian is genuinely strange, occupying a category of its own among the world's waters.
The Geography
The Caspian is the remnant of an ancient sea — the Paratethys — that once connected to the Mediterranean and the Black Sea. Tectonic activity over millions of years isolated the Caspian from the world ocean, and it has been enclosed for several million years. Its surface sits about 28 metres below sea level, making it the lowest part of an internal drainage basin that extends across much of Central Asia. The northern part of the sea is extremely shallow — averaging only about 5 metres deep — and freezes over in winter. The middle Caspian is moderately deep, averaging around 190 metres. The southern Caspian, separated from the middle by the Apsheron Sill, is deep — up to 1,025 metres at its deepest point, deeper than the entire average depth of the Mediterranean. The water is brackish, with average salinity around 1.2 percent (compared to 3.5 percent for ocean water), though salinity varies considerably from near-fresh in the north to about 1.3 percent in the south. The lake's level fluctuates substantially on multi-decade cycles, sometimes by several metres over decades, in ways that scientists still struggle to model accurately.
Lake or Sea?
The Caspian's legal status was undecided for nearly three decades after the collapse of the Soviet Union in 1991. The question matters because international law treats lakes and seas very differently. If the Caspian is a sea, then the United Nations Convention on the Law of the Sea applies, which would give each of the five littoral countries a 12-nautical-mile territorial sea and a 200-mile exclusive economic zone — but with the modest 371,000 km² total surface area, these zones would overlap and create complex shared-jurisdiction problems. If it is a lake, then the legal regime is governed by bilateral agreements between the bordering countries and by general international lake law, which typically calls for either equal sharing or median-line division.
From 1991 until 2018, the five countries argued over the question. The 1921 and 1940 Soviet-Iranian treaties that had governed the lake during the Soviet period treated it as a kind of condominium between the two powers, with Iran and the USSR sharing fishing and navigation rights. The 1991 emergence of three new littoral states (Kazakhstan, Turkmenistan, and Azerbaijan) required a new framework. Negotiations stalled repeatedly. The Caspian Sea Convention, signed by all five states at Aktau in Kazakhstan in August 2018, finally produced a legal settlement: the Caspian is neither a lake nor a sea but a "special legal status," with each state controlling its own 15-nautical-mile coastal zone plus a 10-mile fishing zone, and the central waters open to all five for navigation.
Oil and Gas
The Caspian basin contains some of the world's largest reserves of oil and natural gas. Total proven reserves are estimated at around 50 billion barrels of oil and 9 trillion cubic metres of natural gas, with much larger possible reserves not yet fully developed. The major fields are concentrated in Azerbaijan (Azeri-Chirag-Gunashli, one of the largest oilfields in the world), Kazakhstan (Tengiz and Kashagan, the latter being one of the most expensive single oilfields ever developed at over $50 billion in capital cost), Turkmenistan (mostly natural gas), and Iran (substantially less developed because of sanctions and political constraints). Oil from the Caspian basin reaches world markets primarily through pipelines: the Baku-Tbilisi-Ceyhan pipeline carries Azerbaijani oil to the Mediterranean coast of Turkey, the Caspian Pipeline Consortium line carries Kazakh oil to Russia's Black Sea coast, and various smaller pipelines feed into Russian and Iranian networks. Pipeline disputes have been a recurring source of regional tension.
The Caviar Question
The Caspian Sea is the source of the great majority of the world's wild caviar. Five species of sturgeon — the beluga, Russian, Persian, sevruga, and sterlet — spawn in the rivers feeding the lake and spend their adult lives in its waters. The fish are caught for their roe, which is processed into caviar. The most prized variety is beluga, from the largest sturgeon species; a kilogram of high-grade beluga caviar has retailed for over $10,000 in some markets. The Caspian fishery has been catastrophically overfished and damaged by dam construction (which blocks spawning runs) and by pollution. Wild beluga sturgeon populations have declined by over 90 percent since the 1970s. CITES imposed an export quota system on Caspian caviar in 1998, and a complete ban on wild beluga caviar exports in 2008. Most caviar in international trade today is from farmed sturgeon. Illegal poaching continues at substantial volumes.
Baku and the Cities of the Caspian
Several major cities sit on the Caspian shore. Baku, the capital of Azerbaijan, is the largest at about 2.3 million people and the historic centre of the Caspian oil industry — Baku's first commercial oil wells went into production in the 1840s, and the city was the world's leading oil producer in the early 20th century. Astrakhan in southern Russia sits at the Volga delta and is the historic gateway for Russian trade into Central Asia. Aktau, on the Kazakh coast, was a closed Soviet uranium-processing city and is now a small port and resort town of around 200,000. Turkmenbashi (formerly Krasnovodsk), in Turkmenistan, is the eastern terminus of trans-Caspian shipping. Rasht and Bandar Anzali, on the Iranian coast, are smaller and historically separated from the rest of Iran by mountains; they connect to the Caspian fishery and to limited Iranian-Russian trade routes. The cities collectively form a Caspian-rim community with its own trade and cultural patterns, much of it Russian-language-mediated even where the underlying populations speak Azerbaijani, Turkmen, Kazakh, or Persian.
The Volga
The Caspian receives about 80 percent of its freshwater inflow from a single river — the Volga, which drains roughly one-third of European Russia. The Volga is itself one of the great rivers of the world, with a course of 3,500 kilometres and an enormous drainage basin that includes most of Russia's industrial heartland and many of its largest cities (Moscow indirectly, plus Nizhny Novgorod, Kazan, Samara, Volgograd, and Astrakhan along its banks). The Volga's discharge has been substantially reduced over the past century by dam construction for hydropower and irrigation, which has caused the Caspian's northern reaches to retreat from their historic positions. Volga water also carries significant pollution from upstream industry, which accumulates in the closed Caspian basin and contributes to the decline of the fishery.
The Aral Comparison
The Caspian is sometimes compared with the Aral Sea — another formerly large enclosed Central Asian water body — as a possible cautionary tale. The Aral has lost 90 percent of its volume since 1960 because the rivers feeding it were diverted for cotton irrigation. The Caspian has not had a comparable diversion, partly because the Volga's drainage basin is too large and too politically diverse to allow consolidated diversion, and partly because the Soviet planners who built the cotton system always understood that the Caspian was too important to lose. But the lake's level has dropped by approximately 2 metres since the mid-1990s, primarily because of reduced Volga inflows combined with increased evaporation in the warming climate. Recent projections suggest the lake could lose another 5-15 metres of depth by 2100, which would expose substantial portions of the shallow northern shelf and disrupt ports across the region.
What Comes Next
The Caspian's most pressing problems are environmental. Falling water levels threaten ports and fisheries. Continued pollution from industrial and agricultural runoff threatens the remaining sturgeon populations. Oil extraction creates incidents and slow contamination. Climate change is accelerating evaporation rates faster than rainfall and inflow can compensate. The basin's closed nature means that pollutants accumulate over decades rather than being flushed to oceans. The five littoral states have established cooperation frameworks (the Tehran Convention of 2003 on environmental protection, the 2018 Aktau Convention on the legal status), but practical implementation has been slow. The Caspian is in some ways a slow-motion version of the Aral disaster, with much more time and many more political constraints on responding to it.
