BLACK SEA (or Euxine; anc. Pontus Euxinus),  a body of water lying almost entirely between the latitudes 41º and 45º N., but extending to about 47º N. near Odessa. It is bounded N. by the southern coast of Russia; W. by Rumania, Turkey and Bulgaria; S. and E. by Asia Minor. The northern boundary is broken at Kertch by a strait entering into the Sea of Azov, and at the junction of the western and southern boundary is the Bosporus, which unites the Black Sea with the Mediterranean through the Sea of Marmora and the Dardanelles. The 100-fathom line is about 10 to 20 m. from the shore except in the north-west corner between Varna and Sevastopol, where it extends 140 m. seawards. The greatest depth is 1030 fathoms (1227 Russian fathoms) near the centre, there being only one basin. The steepest incline outside 100 fathoms is to the south-east of the Crimea and at Amastra; the incline to the greater depths is also steep off the Caucasus and between Trebizond and Batum. The conditions that prevail in the Black Sea are very different from those of the Mediterranean or any other sea. The existence of sulphuretted hydrogen in great quantities below 100 fathoms, the extensive chemical precipitation of calcium carbonate, the stagnant nature of its deep waters, and the absence of deep-sea life are conditions which make it impossible to discuss it along with the physical and biological conditions of the Mediterranean proper.
The depths of the Black Sea are lifeless, higher organic life not being known to exist below 100 fathoms. Fossiliferous remains of Dreissena, Cardium and other molluscs have, however, been dredged up, which help to show that conditions formerly existed in the Black Sea similar to those that exist at the present day in the Caspian Sea. According to N. Andrusov, when the union of the Black Sea with the Mediterranean through the Bosporus took place, salt water rushed into it along the bottom of the Bosporus and killed the fauna of the less saline waters. This gave rise to a production of sulphuretted hydrogen which is found in the deposits, as well as in the deeper waters.
Observations in temperature and salinity have only been taken during summer. During summer the surface salinity of the Black Sea is from 1.70 to 2.00% down to 50 fathoms, whereas in the greater depths it attains a salinity of 2.25%. The temperature is rather remarkable, there being an intermediate cold layer between 25 and 50 fathoms. This is due to the sinking of the cold surface water (which in winter reaches freezing-point) on to the top of the denser more saline water of the greater depths. There is thus a minimum circulation in the greater depths causing there uniformity of temperature, an absence of the circulation of oxygen by other means than diffusion, and a protection of the sulphuretted hydrogen from the oxidation which takes place in homologous situations in the open ocean. The temperature down to 25 fathoms is from 78.3º to 46.2º F., and in the cold layer, between 25 and 50 fathoms, is from 46.2º to 43.5º F., rising again in greater depths to 48.2º F.
The Sea of Marmora may be looked upon as an arm of the Aegean Sea and thus part of the Mediterranean proper. Its salinity is comparable to that of the eastern basin of the Mediterranean, which is greater than that of the Black Sea, viz. 4%. Similar currents exist in the Bosporus to those of the Strait of Gibraltar. Water of less salinity flows outwards from the Black Sea as an upper current, and water of greater salinity from the Sea of Marmora flows into the Black Sea as an under-current. This under-current flows towards Cape Tarhangut, where it divides into a left and right branch. The left branch is appreciably noticed near Odessa and the north-west corner; the right branch sweeps past the Crimea, strikes the Caucasian shore (where it comes to the surface running across, but not into, the south-east corner of the Black Sea), and finally disperses flowing westwards along the northern coast of Asia Minor between Cape Jason and Sinope. This current causes a warmer climate where it strikes. So marked is this current that it has to be taken into account in the navigation of the Black Sea.
The Sea of Azov is exceedingly shallow, being only about 6 fathoms in its deepest part, and it is largely influenced by the river Don. Its water is considerably fresher than the Black Sea, varying from 1.55 to 0.68%. It freezes more readily and is not affected by the Mediterranean current.
See N. Andrusov, "Physical Exploration of the Black Sea," in Geographical Journal, vol. i. p. 49.
 The early Greek navigators gave it the epithet of axenus, i.e. unfriendly to strangers, but as Greek colonies sprang up on the shores this was changed to euxinus, friendly to strangers.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)