Europe is a continent that comprises the westernmost part of Eurasia. Europe is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, and the Mediterranean Sea to the south. To the east and southeast, Europe is generally considered as separated from Asia by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways of the Turkish Straits. Yet the borders of Europe—a concept dating back to classical antiquity—are arbitrary, as the primarily physiographic term "continent" also incorporates cultural and political is the world's second-smallest continent by surface area, covering about 10,180,000km2 or 2% of the Earth's surface and about 6.8% of its land area. Of Europe's approximately 50 countries, Russia is by far the largest by both area and population, taking up 40% of the continent, while Vatican City is the smallest. Europe is the third-most populous continent after Asia and Africa, with a population of 739–743 million or about 11% of the world's population.