The Caspian Sea is the largest closed body of water on earth. The Caspian Sea, which is apart from the ocean, contains up to 45% of the water found in all of the world's lakes. The Caspian Sea is -28.42 meters below sea level (year 2022). The length of the sea is 1200 km, the maximum width is 466 km, the minimum width is 204 km, the area is 392 thousand km², the water volume is 79 thousand km³. The border between our republic and the Caspian Sea is 816 km. The area of the Caspian Sea is equal to 44 percent of the total area of all lakes in the world.  Its average depth is 207 meters and the deepest place is 1025 meters (Lankaran trench). The water level there is 28 meters below the ocean's surface. The geographical and morphometric properties of the Caspian Sea are subject to change because to the sea's level's ongoing fluctuation. Thus, the sea level varies in the interval of 30-40 cm by months. The range of sea area change is from 10% to 20%. The number of rivers flowing into the Caspian Sea is more than 130. The Volga River holds the top spot among these rivers. The Caspian Sea and the World Ocean are linked by the Volga River. The Volga River has a special importance in feeding the sea. The total annual flow to the sea through this river is 80%, other rivers - Kura 6%, Ural 5%, Terek, Sulaq and Samur together 4.5%, Safidrud, Shirrud, Gurgan etc. from the territory of Iran 4.0-4.5% does.
            Due to its geographical structure, the climate of the Caspian Sea can be called the biggest climate indicator of our planet. As we mentioned, the Caspian Sea is a closed water basin and has a special hydrological regime. Meteorological conditions and climate over the Caspian Sea and adjacent areas are formed under the influence of atmospheric circulation over the Eurasian continent, the relief of the coastal zone and the water surface. Its considerable length along the meridian determines the diversity of air temperature distribution in its water area. The annual variations of air humidity over the sea water area are similar to the annual variations of air temperature. Climate, water flow, water depth, bottom topography and ruggedness of the coast are the main factors providing the hydrological regime.

The water of the Caspian Sea is of oceanic origin and has an average salinity of 12.85 ‰ (promil) (the average salinity of ocean water is 35 ‰). The low salinity is due to the fact that the sea is closed and fed mainly by river flows. Compared to ocean waters, the amount of carbonates and sulfates in the water of the Caspian Sea is high, and the amount of chlorides is low. The Caspian Sea is divided into two parts according to the salinity of the water.

Salinity levels in the deepest regions of the Caspian Sea range from 12.8 to 12.9 ‰ on average.. The salinity changes slightly with depth, increasing by about 0.1-0.2 ‰. The salinity in the water area of the western shores of the sea is lower than in the water area of the eastern coast. The main reason for this is the flow of the Volga River along the west coast. A similar situation occurs due to the flow of the Kura River in the South Caspian. In winter, salinity in the Middle Caspian increases from 11.0 ‰ in the northwest to 13.4 ‰ in the southeast. In summer, the salinity in both the Middle and South Caspian Seas (on the surface) is the same, 12.8-12.9 ‰. However, salinity increases up to 13 ‰ on the eastern coast of the Middle Caspian and in the central areas. The change of salinity in the Caspian Sea mainly depends on the change of the river flow entering the sea, that is, the water balance. Precipitation, seasons, evaporation from the sea surface, sea level and water exchange between different parts of the sea are of secondary importance in the distribution of salinity over the sea surface.