Propontis

In the Argonautica Propontis is described as a lofty island inside the Propontis, a short distance from the Phrygian mainland with its rich cornfields, sloping to the sea, where an isthmus in front of the mainland is flooded by the waves, so low does it lie.

In reality it is located in the Sea of Marmara that connects the Black Sea to the Aegean Sea, thus separating Turkey's Asian and European parts.

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It is referenced in the following articles

Bithynia
Jason and the Argonauts
Mysia
The Odyssey and the Argonauts