The sea's ancient Greek name Propontis derives from pro- (before) and pontos (sea), deriving from the fact that the Greeks sailed through it to reach the Black Sea, Pontos. In Greek mythology, a storm on Propontis brought the Argonauts back to an island they had left, precipitating a battle where either Jason or Heracles killed King Cyzicus, who mistook them for his Pelasgian enemies.
The surface salinity of the sea averages about 22 parts per thousand, which is slightly greater than that of the Black Sea, but only about two-thirds that of most oceans. The water is much more saline at the sea bottom, averaging salinities of around 38 parts per thousand, similar to that of the Mediterranean Sea. This high-density saline water, like that of the Black Sea, does not migrate to the surface. Water from the Susurluk, Biga (Granicus) and Gonen Rivers also reduces the salinity of the sea, though with less influence than on the Black Sea. With little land in Thrace draining southward, almost all of these rivers flow from Anatolia.
The south coast of the sea is heavily indented, and includes the Gulf of İzmit (Turkish: İzmit Körfezi), the Gulf of Gemlik (Turkish: Gemlik Körfezi) Gulf of Bandırma (Turkish: Bandırma Körfezi) and the Gulf of Erdek (Turkish: Erdek Körfezi). During a storm on December 29, 1999, the Russian oil tanker Volgoneft broke in two in the Sea of Marmara, and more than 1,500 tonnes of oil were spilled into the water.