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The sea stretches nearly 1,200 kilometres (750 mi) from north to south, with an average width of 320 km (200 mi). Its gross coverage is 386,400 km2 (149,200 sq mi) and the surface is about 27 m (89 ft) below sea level. Its main freshwater inflow, Europe's longest river, the Volga, enters at the shallow north end. Two deep basins form its central and southern zones. These lead to horizontal differences in temperature, salinity, and ecology. The seabed in the south reaches 1,023 m (3,356 ft) below sea level, which is the second lowest natural depression on Earth after Lake Baikal (−1,180 m or −3,870 ft). Written accounts from the ancient inhabitants of its coast perceived the Caspian Sea as an ocean, probably because of its saltiness and large size.