The Sea of Azov is the shallowest sea in the world with an average depth of and maximum depth of ; in the bays, where silt has built up, the average depth is about . The sea bottom is also relatively flat with the depth gradually increasing from the coast to the centre. The Sea of Azov is an internal sea with passage to the Atlantic Ocean going through the Black, Marmara, Aegean and Mediterranean seas. It is connected to the Black Sea by the Strait of Kerch, which at its narrowest has a width of and a maximum depth of . The narrowness of the Kerch Strait limits the water exchange with the Black Sea. As a result, the salinity of the Sea of Azov is low; in the open sea it is 10–12 psu, about one third of the salinity of the oceans; it is even lower (2–7 psu) in the Taganrog Bay at the northeast end of the Sea. The long-term variations of salinity are within a few psu and are mostly caused by changes in humidity and precipitation.