Caspian Sea
The wide and endorheic Caspian Sea has a north–south orientation and its main freshwater inflow, the Volga River, enters at the shallow north end. Two deep basins occupy its central and southern areas. These facts lead to horizontal differences in temperature, salinity, and ecology. The Caspian Sea spreads out over nearly 750 miles (1,200 km) from north to south, with an average width of 200 miles (320 km). It covers a region of around 149,200 square miles (386,400 square km)—bigger than the region of Japan—and its surface is about 90 feet (27 meters) below sea level. The sea bed in the southern part reaches as low as 1,023 m (3,356 ft) below sea level, which is the second lowest natural depression on earth after Lake Baikal (−1,180 m, −3,871 ft). The ancient inhabitants of its coast perceived the Caspian Sea as an ocean, probably because of its saltiness and large size.
Sea of Azov
The Maeotian marshes around the mouth of the Tanais River (the present-day Don) were famous in antiquity, as they served as an important check on the migration of nomadic people from the Eurasian steppelands. The Maeotians themselves lived by fishing and farming, but were avid warriors able to defend themselves against invaders. Misled by its strong currents, ancient geographers had only a vague idea of the extent of the sea, whose fresh water caused them to typically label it a "swamp" or a "lake". Herodotus (5th-century BC) judged it as large as the Black Sea, while the Pseudo-Scylax (4th-century BC) thought it about half as large. It was long thought to provide direct communication with the Arctic Ocean. Polybius (2nd-century BC) confidently expected that the strait to the Sea of Azov would close in the near future, presumably due to falling sea-levels. In the 1st-century, Strabo reckoned the distance from the Cimmerian Bosporus (the Strait of Kerch) to the mouth of the Tanais at , a roughly correct figure,
but did not know that its width continuously narrows.