CASPIAN SEA (anc. Mare Caspium or Mare Hyrcanium; Russian, Kaspiyskoe More, formerly Hvalynskoe More; Persian, Darya-i-Khyzyr or Gurzem; Tatar, Ak-denghiz; the Sikim and Jurjan of the ancient Eastern geographers), an inland sea between Europe and Asia, extending from 36° 40' to 47° 20' N. lat., and from 46° 50' to 55° 10' E. long. Its length is 760 m. from N. to S., and its breadth 100 to 280 m., and its area reaches 169,330 sq. m., of which 865 sq. m. belong to its islands. It fills the deepest part of a vast depression, sometimes known as the Aralo-Caspian depression, once an inland sea, the Eurasian Mediterranean or Sarmatian Ocean. At the present time its surface lies 86 ft. below the level of the ocean, or 96.7 ft. according to the Aral-Caspian levelling  and 242.7 ft. below the level of the Aral.
Hydrography and Shores. - The hydrography of the Caspian Sea has been studied by von Baer, by N. Ivashintsev (1819-1871) in 1862-1870, by O. Grimm, N.I. Andrusov (1895), and by J.B. Spindler (1897), N. von Seidlitz and N. Knipovich (1904) since the last quoted date. Its basin is divided naturally into three sections - (1) A northern, forming in the east the Gulf of Mortvyi Kultuk or Tsarevich Bay. This is the shallowest part, barely reaching a depth of 20 fathoms. It is being gradually silted up by the sedimentary deposits brought down by the rivers Volga, Ural and Terek. The western shore, from the delta of the Volga to the mouth of the Kuma, a distance of 170 m., is gashed by thousands of narrow channels or lagoons, termed limans, from 12 to 30 m. in length, and separated in some cases by chains of hillocks, called bugors, in others by sandbanks. These channels are filled, sometimes with sea-water, sometimes with overflow water from the Volga and the Kuma. The coast-line of the Gulf of Mortvyi Kultuk on the north-east is, on the other hand, formed by a range of low calcareous hills, constituting the rampart of the Ust-Urt plateau, which intervenes between the Caspian and the Sea of Aral. On the south this gulf is backed by the conjoined peninsulas of Busachi and Manghishlak, into which penetrates the long, narrow, curving bay or fjord of Kaidak or Kara-su. (2) South of the line joining the Bay of Kuma with the Manghishlak peninsula, in 44° 10' N. lat., the western shore is higher and the water deepens considerably, being over one-half of the area 50 fathoms, while the maximum depth (between 41° and 42° N. lat.) reaches 437 fathoms. This, the middle section of the Caspian, which extends as far as the Apsheron peninsula, receives the Terek and several smaller streams (e.g. Sulak, Samur), that drain the northern slopes of the Caucasus. At Derbent, just north of 42° lat., a spur of the Caucasus approaches so close to the sea as to leave room for only a narrow passage, the Caspiae Pylae or Albanae Portae, which has been fortified for centuries. The eastern shore of this section of the sea is also formed by the Ust-Urt plateau, which rises 550 ft. to 750 ft. above the level of the Caspian; but in 42° N. lat. the Ust-Urt recedes from the Caspian and circles round the Gulf of Kara-boghaz or Kara-bugaz (also called Aji-darya and Kuli-darya). This subsidiary basin is separated from the Caspian by a narrow sandbar, pierced by a strait 1 m. long and only 115 to 170 yds. wide, through which a current flows continuously into the gulf at the rate of 1 to 5 m. an hour, the mean velocity at the surface being 3 m. an hour. To this there exists no compensating outflow current at a greater depth, as is usually the case in similar situations. The area of this lateral basin being about 7100 sq. m., and its depth but comparatively slight (3 to 36 ft.), the evaporation is very appreciable (amounting to 3.2 ft. per annum), and sufficient, according to von Baer, to account for the perpetual inflow from the Caspian. South of the Kara-Boghaz Bay the coast rises again in another peninsula, formed by an extension of the Balkhan Mountains. This marks (40° N. lat.) the southern boundary of the middle section of the Caspian. This basin may be, on the whole, considered as a continuation of the synclinal depression of the Manych, which stretches along the northern foot of the Caucasus from the Sea of Azov. It is separated from (3), the southern and deepest section of the Caspian, by a submarine ridge (30 to 150 fathoms of water), which links the main range of the Caucasus on the west with the Kopet-dagh in the Transcaspian region on the east. This section of the sea washes on the south the base of the Elburz range in Persia, sweeping round from the mouth of the Kura, a little north of the Bay of Kizil-agach, to Astarabad at an average distance of 40 m. from the foot of the mountains. A little east of the Gulf of Enzeli, which resembles the Kara-boghaz, though on a much smaller scale, the Sefid-rud pours into the Caspian the drainage of the western end of the Elburz range, and several smaller streams bring down the precipitation that falls on the northern face of the same range farther to the east. Near its south-east corner the Caspian is entered by the Atrek, which drains the mountain ranges of the Turkoman (N.E.) frontier of Persia. Farther north, on the east coast, opposite to the Bay of Kizil-agach, comes the Balkhan or Krasnovodsk Bay. In the summer of 1894 a subterranean volcano was observed in this basin of the Caspian, in 38° 10' N. lat. and 52° 37' E. long. The depth in this section ranges from 300 to 500 fathoms, with a maximum of 602 fathoms.
Drainage Area and Former Extent. - The catchment area from which the Caspian is fed extends to a very much greater distance on the west and north than it does on the south and east. From the former it is entered by the Volga, which is estimated to drain an area of 560,000 sq. m., the Ural 96,000 sq. m., the Terek 59,000 sq. m., the Sulak 7000 sq. m., the Samur 4250 sq. m.; as compared with these, there comes from the south and east the Kura and Aras, draining the south side of the Caucasus over 87,250 sq. m., and the Sefid-rud and the Atrek, both relatively short. Altogether it is estimated (by von Dingelstedt) that the total discharge of all the rivers emptying into the Caspian amounts annually to a volume equal to 174.5 cub. m. Were there no evaporation, this would raise the surface of the sea 5 ft. annually. In point of fact, however, the entire volume of fresh water poured into the Caspian is only just sufficient to compensate for the loss by evaporation. Indeed in recent times the level appears to have undergone several oscillations. From the researches of Philippov it appears that during the period 1851-1888 the level reached a maximum on three separate occasions, namely in 1868-1869, 1882 and 1885, while in 1853 and 1873 it stood at a minimum; the range of these oscillations did not, however, exceed 3 ft. 6 in. The Russian expedition which investigated the Kara-boghaz in 1896 concluded that there is no permanent subsidence in the level of the sea. In addition to these periodical fluctuations, there are also seasonal oscillations, the level being lowest in January and highest in the summer.
The level of the Caspian, however, was formerly about the same as the existing level of the Black Sea, although now some 86 ft. below it. This is shown by the evidences of erosion on the face of the rocks which formed the original shore-line of its southern basin, those evidences existing at the height of 65 to 80 ft. above the present level. That a rapid subsidence did take place from the higher level is indicated by the fact that between it and the present level there is an absence of indications of erosive energy. There can be no real doubt that formerly the area of the Caspian was considerably greater than it is at the present time. Nearly one hundred and fifty years ago Pallas had his attention arrested by the existence of the salt lakes and dry saline deposits on the steppes to the east of the Caspian, and at great distances from its shores, and by the presence in the same localities of shells of the same marine fauna as that which now inhabits that sea, and he suggested the obvious explanation that those regions must formerly have been covered by the waters of the sea. And it is indeed the fact that large portions of the vast region comprised between the lower Volga, the Aral-Irtysh water-divide, the Dzungarian Ala-tau, and the outliers of the Tian-shan and Hindu-kush systems are actually covered with Aralo-Caspian deposits, nearly always a yellowish-grey clay, though occasionally they assume the character of a more or less compact sandstone of the same colour. These deposits attain their maximum thickness of 90 ft. east of the Caspian, and have in many parts been excavated and washed away by the rivers (which have frequently changed their beds) or been transported by the winds, which sweep with unmitigated violence across those wide unsheltered expanses. The typical fossils unearthed in these deposits are shells of species now living in both the Caspian and the Aral, though in the shallow parts of both seas only, namely (according to Ivan V. Mushketov [1850-1902]) Cardium edule, Dreissena polymorpha, Neritina liturata, Adacna vitrea, Hydrobia stagnalis, in the Kara-kum desert, and Lithoglyphus caspius, Hydrobia stagnalis, Anodonta ponderosa and the sponge Metchnikovia tuberculata, in the Kizil-kum desert. The exact limits of the ancient Aralo-Caspian sea are not yet settled, except in the north-west, where the Ergeni Hills of Astrakhan constitute an unmistakable barrier. Northwards these marine deposits are known to exist 80 m. away from Lake Aral, though they do not cross the Aral-Irtysh water-divide, so that this sea will not probably have been at that time connected with the Arctic, as some have supposed. The eastern limits of these deposits lie about 100 m. from Lake Aral, though Severtsov maintained that they penetrate into the basin of Lake Balkash. Southwards they have been observed without a break for 160 m. from Lake Aral, namely in the Sary-kamysh depression (the surface of which lies below the level of the Caspian) and up the Uzboi trench for 100 m. from the latter sea. How far they reach up the present courses of the Oxus (Amu-darya) and Jaxartes (Syr-darya) is not known. Hence, it is plain that in late Tertiary, and probably also in Post-Tertiary, times the Aralo-Caspian Sea covered a vast expanse of territory and embraced very large islands (e.g. Ust-Urt), which divided it into an eastern and a western portion, communicating by one or two narrow straits only, such as on the south the Sary-kamysh depression, and on the north the line of the lakes of Chumyshty and Asmantai. More than this, the Caspian was also, it is pretty certain, at the same epoch, and later, in direct communication with the Sea of Azov, no doubt by way of the Manych depression; for in the limans or lagoons of the Black Sea many faunal species exist which are not only identical with species that are found in the Caspian, but also many which, though not exactly identical, are closely allied. As examples of the former may be named - Archaeobdella, Clessinia variabilis, Neritina liturata, Gmelina, Gammarus moeoticus, Pseudocuma pectinata, Paramysis Baeri, Mesomysis Kowalevskyi and M. intermedia, Limnomysis Benedeni and L. Brandti, and species of the ichthyological fauna Gobius, Clupea and Acipenser; while as illustrating the latter class the Black Sea contains Dreissenia bugensis (allied to D. rostriformis and D. Grimmi), Cardium ponticum (to C. caspium), C. coloratum (to Monodacna edentula), Amphicteis antiqua (to A. Kowalevskyi) and Bythotrephes azovicus (to B. socialis).
In the opinion of Russian geologists the separation of the Caspian from the great ocean must have taken place at a comparatively recent geological epoch. During the early Tertiary age it belonged to the Sarmatian Ocean, which reached from the middle Danube eastwards through Rumania, South Russia, and along both flanks of the Caucasus to the Aralo-Caspian region, and westwards had open communication with the great ocean, as indeed the ancient geographers Eratosthenes, Strabo and Pliny believed it still had in their day. This communication began to fail, or close up presumably in the Miocene period; and before the dawn of Pliocene times the Sarmatian Ocean was broken up or divided into sections, one of which was the Aralo-Caspian sea already discussed. During the subsequent Ice Age the Caspian flowed over the steppes that stretch away to the north, and was probably still connected with the Black Sea (itself as yet unconnected with the Mediterranean), while northwards it sent a narrow gulf or inlet far up the Volga valley, for Aralo-Caspian deposits have been observed along the lower Kama in 56° N. lat. Eastwards it penetrated up the Uzboi depression between the Great and Little Balkhan ranges, so that that depression, which is strewn (as mentioned above) with Post-Tertiary marine deposits, was not (as is sometimes supposed) an old bed of the Oxus, but a gulf of the Caspian. After the great ice cap had thawed and a period of general desiccation set in, the Caspian began to shrink in area, and simultaneously its connexions with the Black Sea and the Sea of Aral were severed.
Fauna. - The fauna of this sea has been studied by Eichwald, Kowalevsky, Grimm, Dybowski, Kessler and Sars. At the present time it represents an intermingling of marine and fresh-water forms. To the former belongs the herring (Clupea), and to the latter, species of Cyprinus, Perca and Silurus, also a lobster. Other marine forms are Rhizopoda (Rotalia and Textillaria), the sponge Amorphina, the Amphicteis worm, the molluscs Cardium edule and other Cardidae, and some Amphipods (Cumacea and Mysidae,), but they are forms which either tolerate variations in salinity or are especially characteristic of brackish waters. But there are many species inhabiting the waters of the Caspian which are not found elsewhere. These include Protozoa, three sponges, Vermes, twenty-five Molluscs, numerous Amphipods, fishes of the genera Gobias, Benthophilus and Cobitis, and one mammal (Phoca caspia). This last, together with some of the Mysidae and the species Glyptonotus entomon, exhibits Arctic characteristics, which has suggested the idea of a geologically recent connexion between the Caspian and the Arctic, an idea of which no real proofs have been as yet discovered. The Knipovich expedition in 1904 found no traces of organic life below the depth of 220 fathoms except micro-organisms and a single Oligochaete; but above that level there exist abundant evidences of rich pelagic life, more particularly from the surface down to a depth of 80 fathoms.
Fisheries. - No other inland sea is so richly stocked with fish as the Caspian, especially off the mouths of the large rivers, the Volga, Ural, Terek and Kura. The fish of greatest economic value are sturgeon (four species), which yield great quantities of caviare and isinglass, the herring, the salmon and the lobster. The annual catch of the entire sea is valued at an average of one million sterling. Some 50,000 persons are engaged in this industry off the mouth of the Volga alone. Seals are hunted in Krasnovodsk Bay.
Salinity. - The proportion of salt in the water of the Caspian, though varying in different parts and at different seasons, is generally much less than the proportion in oceanic water, and even less than the proportion in the water of the Black Sea. In fact the salinity of the Caspian is only three-eights of that of the ocean. In the northern section, which receives the copious volumes brought down by the Volga, Ural and Terek, the salinity is so slight (only 0.0075% in the surface layers) that the water is quite drinkable, its specific gravity being not higher than 1.0016. In the middle section the salinity of the surface layers increases to 0.015%, though it is of course greater along the shores. The concentration of the saline ingredients proceeds with the greatest degree of intensity in the large bays on the east side of the sea, and more especially in that of Kara-boghaz, where it reaches 16.3% (Spindler expedition). The bottom of this almost isolated basin is covered for an area of 1300 sq. m. with a deposit of Epsom salts (sulphate of magnesia), 7 ft. thick, amounting to an estimated total of 1,000,000,000 tons. While the proportion of common salt to sulphate of magnesia is as 11 to 1 in the water of the Black Sea and as 2 to 1 in the Caspian water generally, it is as 12.8 to 5.03 in the Kara-boghaz. The salinity of the surface water of the southern section of the Caspian averages 1.5%.
Climate. - The temperature of the air over the Caspian basin is remarkable for its wide range both geographically and seasonally. The January isotherm of 15° F. skirts its northern shore; that of 40° crosses its southern border. But the winter extremes go far below this range: during the prevalence of north-east winds the thermometer drops to -20°, or even lower, on the surrounding steppes, while on the Ust-Urt plateau a temperature of -30° is not uncommon. Again, the July isotherm of 75° crosses the middle section of the Caspian, nearly coinciding with the January isotherm of 25°, while that of 80° skirts the southern shore of the sea, nearly coinciding with the January curve of 40°, so that the mean annual range over the northern section of the sea is 60° and over the southern section 40°. The former section, which is too shallow to store up any large amount of heat during the summer, freezes for three or four months along the shores, effectually stopping navigation on the lower Volga, but out in the middle ice appears only when driven there by northerly winds.
The prevalent winds of the Caspian blow from the south-east, usually between October and March, and from the north and north-west, commonly between July and September. They sometimes continue for days together with great violence, rendering navigation dangerous and driving the sea-water up over the shores. They also, by heaping up the water at the one end of the sea or the other, raise the level temporarily and locally to the extent of 4 to 8 ft. The currents of the Caspian were investigated by the Knipovich expedition; it detected two of special prominence, a south-going current along the west shore and a north-going current along the east shore. As a consequence of this the temperature of the water is higher on the Asiatic than on the European side. The lowest temperature obtained was 35°.24 on the bottom in shallow water, the highest 70°.7 on the surface. But in March the temperature, as also the salinity, was tolerably uniform throughout all the layers of water. Another interesting fact ascertained by the same expedition is that the amount of oxygen contained in the water decreases rapidly with the depth: off Derbent in the middle section of the sea the amount diminished from 5.6 cc. per litre at a depth of 100 metres (330 ft.) to 0.32 cc. per litre at a depth of 700 metres (say 2300 ft.). At the same spot samples of water drawn from the bottom were found to contain 0.3 cc. of sulphuretted hydrogen per litre. In the southern section of the sea the decrease is not so rapid. In this latter section Spindler ascertained in July 1897 that the temperature of the surface water 60 m. from Baku was 72.9°, but that below 10 fathoms it sank rapidly, and at 200 fathoms and below it was constant at 21.2°.
Navigation. - The development of the petroleum industry in the Apeshron peninsula (Baku) and the opening (1886) of the Transcaspian railway have greatly increased the traffic across the Caspian Sea. A considerable quantity of raw cotton is brought from Ferghana by the latter route and shipped at Krasnovodsk for the mills in the south and centre of Russia, as well as for countries farther west. And Russia draws her own supplies of petroleum, both for lighting and for use as liquid fuel, by the sea route from Baku. Other ports in addition to those just mentioned are Astrakhan, on the Volga; Petrovsk, Derbent and Lenkoran, on the west shore; Enzeli or Resht, and Astarabad, on the Persian coast; and Mikhailovsk, on the east coast. The Russians keep a small naval flotilla on the Caspian, all other nations being debarred from doing so by the treaty of Turkmanchai (1828).
At various times and by various persons, but more particularly by Peter the Great, the project has been mooted of cutting a canal between the Volga and the Don, and so establishing unrestricted water communication between the Caspian and the Black Sea; but so far none of these schemes has taken practical shape. In 1900 the Hydrotechnical Congress of Russia discussed the plan of constructing a canal to connect the Caspian more directly with the Black Sea by cutting an artificial waterway about 22 ft. deep and 180 ft. wide from Astrakhan to Taganrog on the Sea of Azov.
See works quoted under Aral; also von Baer, "Kaspische Studien," in Bull. Sci. St-Pétersbourg (1855-1859), and in Erman's Archiv russ. (1855-1856); Radde, Fauna und Flora des sudwestlichen Kaspigebietes (1886); J.V. Mushketov, Turkestan (St Petersburg, 1886), with bibliographical references; Ivashintsev, Hydrographic Exploration of the Caspian Sea (in Russian), with atlas (2 vols., 1866); Philippov, Marine Geography of the Caspian Basin (in Russian, 1877); Memoirs of the Aral-Caspian Expedition of 1876-1877 (2 vols, in Russian), edited by the St Petersburg Society of Naturalists; Andrusov, "A Sketch of the Development of the Caspian Sea and its Inhabitants," in Zapiski of Russ. Geog. Soc.: General Geog. vol. xxiv.; Eichwald, Fauna Caspio-Caucasica (1841); Seidlitz, "Das Karabugas Meerbusen," in Globus, with map, vol. lxxvi. (1899); Knipovich, "Hydrobiologische Untersuchungert des Kaspischen Meeres," in Petermanns Mitteilungen, vol. l. (1904); and Spindler, in Izvestia of Russ. Geog. Soc. vol. xxxiv.
(P. A. K.; J. T. Be.)
 By the triangulation of 1840 its level was found to be 84 ft. below the level of the Black Sea. The Caucasus triangulation of 1860-1870 gave 89 ft.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)