[citation needed] In Greek mythology, a storm on the Propontis brought the Argonauts back to an island they had left, precipitating a battle in which either Jason or Heracles killed King Cyzicus, who had mistaken them for his Pelasgian enemies.

The surface salinity of the Marmara averages about 22 parts per thousand, which is slightly more than that of the Black Sea, but only about two-thirds that of most oceans.

The inhabited islands are readily accessible by ferry from both the European and Asian shores of İstanbul and the entire archipelago forms part of the conurbation.

These islands lie within Balıkesir province and are most readily accessible from Tekirdağ in Thrace or Erdek on the southern shore of the Sea of Marmara.

In high summer additional ferries travel to Avşa and Marmara Islands from the centre of İstanbul to facilitate a growing tourist trade.

The North Anatolian Fault runs under the sea and has triggered several major earthquakes, such as the Izmit and Düzce in August and November 1999 respectively.

One of the most serious environmental problems in the Marmara Sea in recent years has been mucilage, a thick, sticky substance that appears for up to six months.