A mediterranean sea, centered at approximately 35° E and 44° N, that is the world's largest inland water basin. It has an area of about 461,000 km² and a volume of 537,000 km³ with a mean depth of around 1200-1300 m, although depths greater than 2000 m are common in the central basin. The western part of the Black Sea is a wide shelf that gradually narrows to the south and breaks at around 100-150 m. In the rest of the basin the shelf doesn't exceed 10-15 km in width. It is connected to the Marmara Sea via the narrow (760 m wide) and shallow (27. 5 m maximum depth) Bosporus Strait, and further connects to the Mediterranean Sea via the long and narrow Dardanelles. It is also connected to the Sea of Azov to the north.

The Black Sea is a dilution basin due to a large freshwater input from the Danube, Dniester, Dnieper, Severskiy Donets and Don rivers (350 km³/yr). The flow through the Bosporus comprises a surface flow of low salinity water towards the Mediterranean (260 km³/yr) and an underlying return flow of salty Mediterranean water (120 km³/yr). Precipitation (140 km³/yr) and evaporation (350 km³/yr) close the freshwater budget. The volume averaged salinity is 22, with surface salinities in the central part ranging from 16-18 and increasing to 21-22. 5 at depths greater than 150-200 m. The surface temperatures range from 25° C in the summer to 6-8° C in the open sea, with the northwestern part and the Sea of Azov covered with ice during the winter. The deep water is 8-9° C year round. The upper 50 m are saturated with oxygen, the content of which diminishes until, at a depth of 150-200 m, hydrogen sulfide appears and renders the lower regions uninhabitable.

The most remarkable circulation feature is the cyclonicically meandering Rim Current, the interior of which is formed either by one elongated cell covering the entire basin or by two separate cyclonic cells occupying the western and eastern halves of the basin. The interior of the Western and Eastern Gyres contains a number of recurring cyclonic mesocale eddies. These are in contact with each other by a recurrent anticyclonic eddy called the Central Basin Eddy, a recurrent feature observed to form via the merging of two anticyclonic eddies pinched off from the baroclinically unstable Rim Current southeast of Crimea and off Cape Sinop.

The upper layer flow field also consists of several mesoscale eddies distributed along the periphery of the basin. The two most pronounced and persistent of such are the anticylonic Batumi Eddy in the southeastern corner of the basin and the anticyclonic Sevastapol Eddy in the continental shelf topography of the Danube Fan, west of the Crimean Peninsula. The latter has also been reported in the literature as the Trabzon Eddy. Two other quasi-permanent anticyclonic eddies are found along the Anatolian (Turkish) coastal belt. They are situated off the Sakarya and Kizilirmak Rivers and given their names. Another quasi-permanent anticyclonic feature is the Bosphorous Eddy located northwest of the Bosphorous-Black Sea junction. Two other recurrent coastal anticyclonic eddies have been identified between Sakarya Canyon and Cape Sinop. Along the northern coast, the anticyclonic Crimean and Causasian Eddies are the most pronounced mesoscale features, with the Kali-Akra Eddy a recurrent feature to the north of the Bosphorous Eddy.

The intermediate depth circulation is characterized by the disappearance of the Rim Current, the shifting of eddy centers, coalescence of eddies, persistence of some features for the whole water column but changes with depth in the structure of others, and more organized and large sub-basin features.