The border with Europe starts with the coast of the eastern Mediterranean, even though Turkey in the Near East extends partly into the Aegean Islands and includes Istanbul on the European side of the Bosphorus. On the north the boundary between the continents of Asia and Europe is commonly regarded as running through the Dardanelles, the Sea of Marmara, the Bosporus, the Black Sea, the Caucasus Mountains, the Caspian Sea, the Ural River to its source, and a long border generally following the eastern side of the Ural Mountains to the Kara Sea, Russia. The Arctic Ocean is the northern border. The Bering Straits divide Asia from North America.