Sea of Marmara

The Sea of Marmara, also called the Marmara Sea or Propontis, is a small inland sea located entirely within Turkey. It connects the Black Sea to the Aegean Sea through the Bosporus and Dardanelles straits, dividing the European and Asian parts of the country. It is recognized as the smallest sea in the world, covering an area of 11,350 square kilometers (4,380 square miles), with dimensions of about 280 kilometers by 80 kilometers (174 miles by 50 miles). Its deepest point reaches 1,370 meters (4,490 feet).

The sea takes its name from Marmara Island, the largest island along its southern coast, which is rich in marble—the Greek word for marble is "mármaron." In ancient times, it was known as Propontis, from the Greek "pro" (in front of) and "pontos" (sea), referring to the Black Sea, as Greek sailors passed through it to reach that larger body of water. According to Greek mythology, a storm on the Propontis forced the Argonauts back to an island they had left, leading to a battle where either Jason or Heracles killed King Cyzicus, who had mistaken them for his Pelasgian enemies.

The International Hydrographic Organization sets the sea’s boundaries: on the west, the Dardanelles limit of the Aegean Sea (a line from Kum Kale to Cape Helles); on the northeast, a line from Cape Rumili to Cape Anatoli. The southern coastline is heavily indented, featuring the Gulfs of İzmit, Gemlik, Bandırma, and Erdek. The sea’s surface salinity averages about 22 parts per thousand—slightly higher than the Black Sea but only about two-thirds that of most oceans. Bottom waters are much saltier, averaging around 38 parts per thousand, similar to the Mediterranean, and this dense saline water does not rise to the surface, much like in the Black Sea. Rivers such as the Susurluk, Biga (Granicus), and Gönen, which mostly flow from Anatolia, reduce salinity, though less effectively than in the Black Sea, as little drainage comes from Thrace.

Two main island groups lie in the Sea of Marmara. To the north are the Princes’ Islands, an archipelago of inhabited islands—Kınaliada, Burgazada, Heybeliada, Büyükada, and Sedef Adası—plus several uninhabited ones like Sivriada, Yassıada, Kaşıkadası, and Tavşanadası. These are easily reached by ferry from both the European and Asian shores of Istanbul and are part of the city’s urban area. To the south are the Marmara Islands, an archipelago including Marmara Island itself and three other inhabited islands—Avşa, Paşalimanı, and Ekinlik—along with 17 mostly uninhabited islands, such as İmralı, a prison island that has held PKK leader Abdullah Öcalan since 1999. These islands belong to Balıkesir Province and are most accessible from Tekirdağ in Thrace or Erdek on the southern shore; in summer, extra ferries run from central Istanbul to Avşa and Marmara Islands for tourism. A few individual islands, like Koç Adası off Tuzla, are privately owned, in this case by the Koç family.

The North Anatolian Fault runs beneath the sea, causing major earthquakes, including the İzmit and Düzce quakes in August and November 1999. The August event is often called the Marmara Earthquake because its epicenter was under the sea, and the worst-hit areas along the coast also experienced a tsunami. On December 29, 1999, a storm broke the Russian oil tanker Volgoneft-248 in two, spilling over 1,500 tonnes of oil. Major Turkish cities, especially Istanbul, surround the Marmara Sea, and despite its small size, the basin houses about one-third of the country’s population. Rapid growth and uncontrolled construction have heavily stressed the sea; for years, waste was dumped with little or no treatment, exceeding the sea’s capacity. Many species have vanished, and fishing was banned in the “East Bay” for years due to severe pollution. A recent serious issue is mucilage—a thick, sticky substance that appeared from January to June in 2021, causing habitat loss, economic problems, and disrupting fishing and other ecosystem services.