The border between Asia and Europe has historically been determined by Europeans[which?] only.[dubious – discuss] The original distinction between the two was made by the ancient Greeks. They used the Aegean Sea, the Dardanelles, the Sea of Marmara, the Bosporus, the Black Sea, the Kerch Strait, and the Sea of Azov as the border between Asia and Europe. The Nile was often used as the border between Asia and Africa (then called Libya), although some Greek geographers suggested the Red Sea would form a better boundary. Darius' canal between the Nile and the Red Sea introduced considerable variation in opinion. Under the Roman Empire, the Don River emptying into the Black Sea was the western border of Asia. It was the northernmost navigable point of the European shore. In the 15th century the Red Sea became established as the boundary between Africa and Asia, replacing the Nile.