The largest lake of the world, Caspian sea (land-blocked body of water), covers an area of 438,000 sq. km. and is located between Iran and countries such as the Republics of Qazaqistan, Turkemanistan, and Azarbayjan.

Its southern coast which extends from the Hosseinqoli Gulf to Astara is in the jurisdiction of Iran. In past this lake was known as Heerkani, and in foreign texts and maps was called 'Caspian'.
The average depth of this lake in the north is 6 m. and to the west and south west regions is at the most 1,000 m. The coast of this lake is sandy and low-lying, with the Gulfs of Qarah Boqaz and Gorgan to the north east and southeast respectively.
The latter is segregated from the Caspian Sea due to the presence of the Miankaleh Peninsula.