The Sea of Marmara (Turkish: Marmara Denizi, Greek: Θάλασσα του Μαρμαρά or Προποντίς, Bulgarian: Мраморно море,Hungarian: Márvány-tenger(Marble sea)), also known as the Sea of Marmora or the Marmara Sea, and in the context of classical antiquity as Propontis (Greek: Προποντίς), is the inland sea that connects the Black Sea to the Aegean Sea, thus separating Turkey's Asian and European parts. The Bosporus strait connects it to the Black Sea and the Dardanelles strait to the Aegean. The former also separates Istanbul into its Asian side and European side. The Sea has an area of 11,350 km² (280km x 80km) with the greatest depth reaching 1 370 m.
The 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. However, 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 itself, does not migrate to the surface. Water from the Granicus, Susurluk, Biga 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.
There are two major island groups known as the Prince's and Marmara islands. The latter group is rich in sources of marble and gives the sea its name (Greek marmaro, marble). Alternatively, the name may come from the Indo-European, compare Hittite marmar(r)a, from *mori 'inland body of water'. A notable island located in this sea is İmralı, where Abdullah Öcalan is imprisoned.
The sea's ancient Greek name Propontis derives from pro (before) and pont- (sea), deriving from the fact that the Greeks sailed through it to reach the Black Sea. 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.