Sea or bay
Sinus Heraoticus
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The waters around Ereğli, a town whose name still echoes an ancient Heraclea, are probably what the map meant by Sinus Heraoticus, a bay named after that city. The name has almost entirely faded: only its first letter, S, can still be read, while older copies show a few more letters or the full form. That the first word was sinus, a bay, seems plausible, but the second part turns up in no ancient source.