Although long called a continent, in many physical ways Europe is but a great western peninsula of the Eurasian landmass. Its eastern limits are arbitrary and are conventionally drawn along the water divide of the Ural Mountains, the Ural River, the Caspian Sea, and the Caucasus watershed to the Black Sea. On all other sides Europe is surrounded by salt water. The stretches of water between the Mediterranean and Black seas, however, are narrow, and it is possible to swim across the Bosporus to Asia Minor. Not even the Mediterranean Sea is a real physiographic boundary inasmuch as the Atlas Mountains of northwestern Africa are structurally part of the great Tethys geosyncline. Of the oceanic islands of Franz Josef Land, Spitsbergen (Svalbard), Iceland, and the Azores, only Iceland is regarded, mainly on historical grounds, as an integral part of Europe; thus the northwestern boundary is drawn along the Danish Strait.