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The Bosporus Strait, the Sea of Marmara, and the Dardanelles Strait.
The Bosporus is a narrow, 20-mile long strait connecting the Mediterranean and Sea of Marmara to the Black Sea.
It is very close to, but not actually on the Black Sea. It is on the Bosphorus which is a strait linking the Black Sea and the Sea of Marmara which eventually connects through the Dardanelles to the Aegean Sea.
The Strait of Bosporus links the Black Sea and the Sea of Marmara
Bosporus strait
The Bosporus Strait
The Mediterranean Sea The Black Sea The Aegean Sea The Sea of Marmara
The Strait of Bosporus connects the Black Sea to its north with the Sea of Marmara to its south.
The Bosphorus (also called the Istanbul Strait) and the Dardinelles
It's not actually a river; it's a strait named the Bosporus that connects the Black Sea to the Sea of Marmara which then drains into another strait called the Dardanelles and then into the Mediterranean Sea.
There is no sea the borders both the Mediterranean and Black Seas. In order (by water) to get from one sea to the other, you need to pass through the Aegean Sea, the Dardanelles Strait, the Sea of Marmara, and the Bosporus Strait.
Bosporus